Help me pick a prop and gear for my 30' Sisu- Very, very, very long

[Toolate]

Please feel free to tell me I'm a jerk but its a gas engine. I have been doing some homework around here on this subject and there is an incredible wealth of info on this site if you search long enough (thank you to all who contribute, again . I would also like to preface this by saying that I will be working with a prop shop to sort this out and also that I understand that it is not as much a science as much as it is something that is learned by experience and through trial and error.

So, the engine will probably be a crusader 383 rated at 375 hp and max rpm is actually 5000. I would like to cruise at 3000 to about 3300 max just to keep the noise down if possible. I think the engine is making about 380lb-ft of torque and 250 hp at this rpm. Somewhere in there anyway.

Also think the gear will be 2.57:1 so prop rpm will be between 1200 and 1300 rpm at this engine rpm which seems slow to me. There is a 2.1:1 reduction gear available as well which would speed the prop up a bit- 1500-1650 prop rpm- so please tell me if you think I should go one way or another. Hoping to cruise 16kts minimum and be under 10gph.

Boat currently has a 19" prop but I don't know the pitch (it wasn't marked anywhere) and there is not room to go much larger in my opinion- maybe another inch or 2 at the very most. Right now, she will make 9kts at 2500rpm with 1.91:1 reduction so prop speed is 1308 and 11.7kts at 3000 so prop speed is 1570 and 13.2 at 3250 so prop rpm is 1700. If I project this out to 16kts, it means I would have to spin the current prop at about 2200 rpm which would mean 4620 engine rpm with the 2.1:1 gear and 5650 engine rpm with the 2.57:1 gear (too high).

Based on the info above, I will need a prop with more pitch to achieve 16kts if I go with the 2.1 reduction since the boat goes about 11.5kts at about 1500 prop rpm now and the 2.1 reduction would give me about 1500 prop rpm at 3000 engine rpm. Not sure how much or even if this is the way to look at it? Trying to gain more than 2kts by adding pitch seems like too much to me for some reason. Seems like I will need to spin a prop faster to achieve the speeds I want which either means higher rpms which I would like to avoid or a higher (numerically lower) gear.

Captainlarry84 offered this info in a post about a month ago- I saved it:

"Efficiency: The efficiencyof the propeller is what we strive for. 100% efficiency means that as stated above 1 of pitch per revolution equals 1 of forward movement. However 100 % is nearly impossible to achieve. It can however be improved if all of the above factors are carefully planned during your propeller selection.
In order to see just how efficient your current propeller is doing, here is a simple formula which will give you a ball park figure of efficiency loss do to slippage.

RPMs divided by Reduction Gear Ratio X=s Pitch X=s 60
(minutes) divided by 12 (inches per foot) divided by
5280 (feet in a mile) ==s MPH Divided by 1.15 =s Knots

Example:
Test vessel has a maximum RPM of 3000 revolutions it has a 2:0 reduction gear the propeller is a 19 X 19 size and the vessel does 20 knots. Based on this information we can compute the boats propeller efficiency with no other information needed.

3000 RPMs divided by 2 ==s 1500 X 19 ==s 28500 X=s 60 ==s 1710000 divided by 12 ==s 142500 divided by 5280 ==s 26.98 divided by 1.15 = 23.46 Knots

Therefore the above vessel should go 23.46 knots with 100 % propeller efficiency.
The boat however only goes 20 knots. Next we divide 20 into 23.46 =s .86.
20 knots is 86% of 23.46 knots therefore the efficiency of this boats propeller is
86 %."

Great info Larry- on my boat, if I use the 5000 rpm max for this engine, 2.57 gear and a 19x19 prop, is theoretically going to run at 30 knots (!). If I switch to the 2.1:1 gear the theoretical speed is 37kts (!, !). If I use 75% efficiency for my fat rudder and thick bottom paint these numbers leave me at 22kts/28kts at WOT for the two transmissions respectively. To my inexperienced eye, I would go with a 2.57 gear and a prop with a little less pitch so maybe a 19x18 or 19x17 to get the most power and torque out of the gear/engine/prop combination and avoid overloading the engine. On the other hand, this engine will have serious balls so maybe just leave it square and see what happens.

There is a member (Dthompson004) here with the same engine I am thinking of with 2.57:1 reduction and a 20x19 wheel and he as stated that he gets 14kts at 3200 and 21kts at 4100. This seems like a huge range to me (7kt increase in 900 rpm) and I wonder if he is overpropped because the boat is so fast at only 4100 on an engine rated for 5000. This would put him at around 25-27kts WOT which seems scary fast for this old boat. Mr. Thompson, have you run her up to WOT yet? Think we might both learn a little something from the results of that if you would.

Trying to make sense of Larry's math and Dthompsons real world example- there are close if you consider the relatively poor efficiency that I am sure these hulls deliver.

My final thought is that I may have an outdated view of gas engines because they are meant to spin faster today than they were in the past- its just that I don't want to listen to it go any faster if I don't have to. Do I have to? How low can I gear this engine at 3200 and not overload it and still get my 16kt cruise?

Told you it was a long post.:|

 

[Toolate]

Gurryman, the shaft is 1 1/2" and prop is 4 blade, current engine wont spin up past 3400 so its hard to test. If its more than 10gph, so be it but I would like to make a guess at what it is going to be.

Thompsons boat info:
2.57 : 1. spinning 20 x 19 prop at
3200 engine = 1245 prop = 14kts
4100 engine = 1595 prop = 21

Another boat I just learned about (diesel):
2:1 engine at 2400 rpm spinning
20 x 21 DCQ 4 blade at 1200 rpm= 16kts

This is what I "know". That last post was a little embarrassing really- so long. I wrote it in 3 separate stints at my desk. Sorry.

 

[Toolate]

Here she is- the cage has been removed so that is not slowing me down.

 

[Toolate]

Think about 1060 lbs.

 

[BillD]

OK, refresh my (our memory).

1. What engine is in the boat now?
2. What is the rated WOT rpm and hp for the engine in the boat now?
3. You stated the engine turns 3400 WOT. Is that accurate with a photo tach or reading the analogue gauge ??
4. What is the gear ratio in the boat now?
5. You have a 19" prop now correct?...not sure of the pitch correct?
6. Are you running the boat now with that jail cell around the prop or is it removed ??

OK, get these answered as best you can.

If you are planning on repowering over the winter you need to one more "known" value. The pitch of the prop on the boat now. If you don't know it I (we) have a solution. A sea trial BEFORE you pull the boat out of the water with a photo tach and gps for speed in hand.

Answer the questions, we'll help you get dialed in

FWIW, Bill D

 

[Toolate]

OK, refresh my (our memory).

1. What engine is in the boat now?
2. What is the rated WOT rpm and hp for the engine in the boat now?
3. You stated the engine turns 3400 WOT. Is that accurate with a photo tach or reading the analogue gauge ??
4. What is the gear ratio in the boat now?
5. You have a 19" prop now correct?...not sure of the pitch correct?
6. Are you running the boat now with that jail cell around the prop or is it removed ??

OK, get these answered as best you can.

If you are planning on repowering over the winter you need to one more "known" value. The pitch of the prop on the boat now. If you don't know it I (we) have a solution. A sea trial BEFORE you pull the boat out of the water with a photo tach and gps for speed in hand.

Current engine is 318 Chrysler gas rated at 235hp at WOT that I think is supposed to be around 3700 but the engine wont turn past 3400 for some reason (rpms just fall off, engine stumbles, not sure if its fuel/air/spark or what) so that is what I have.

Using stock tach so that could be totally wrong- great point. Where can I get a good photo tach for a reasonable price?

Gear ratio is 1.91:1 currently.

Prop is 19" no idea of pitch (you couldnt tell from the pic? haha)

Propeller has be released from jail after serving 30 years.

Looking forward to any help anyone is willing to offer. What is the reward or punishment for longest post ever?

 

[RyanCT]

Harbor freight has a photo tach for $35......not sure how long it will hold up.

Could also find them at your local auto part store.

 

[BillD]

Current engine is 318 Chrysler gas rated at 235hp at WOT that I think is supposed to be around 3700 but the engine wont turn past 3400 for some reason (rpms just fall off, engine stumbles, not sure if its fuel/air/spark or what) so that is what I have.

Using stock tach so that could be totally wrong- great point. Where can I get a good photo tach for a reasonable price?

Gear ratio is 1.91:1 currently.

Prop is 19" no idea of pitch (you couldnt tell from the pic? haha)

Propeller has be released from jail after serving 30 years.

Looking forward to any help anyone is willing to offer. What is the reward or punishment for longest post ever?

To begin with, stop beating yourself up !!!
Trying to get a new motor gear prop dialed in to a particular hull requires "asking for help". Doesn't matter whether it's gas or diesel.
You haven't asked anything most of us here haven't asked!

OK, nothing complicated here. You'll be dealing with "known values" when you are ready to do a re-power with gear and prop.

What you'll end up with for speed will be a function of the hydrodynamics of the 30 SISU, the finished weight and the power you install.

When you get the data collected on your boat and what's in/on it now pushing the hull a prop shop will get ya close to what speed you'll end up with the engine you choose.

I would not get to much ahead of yourself.

Get yourself a photo tach..about 40-50 bucks online.

I found an old post on another forum:

The Chrysler line of engines
"The Chrysler line of engines in boat applications has an excellent history. Spec RPM for the 318 at WOT is 4000 RPM. Condition of bottom, tune of engine and weight carried all contribute to RPM obtainable. In good tune with a clean bottom and normal load you pick a prop that will allow the engine to achieve spec WOT RPM. Running at WOT and not achieving rated RPM is extremely hard on the engine and should be avoided."

Start here. Get the photo tach. Get me an "accurate" WOT @ the dock IN NEUTRAL !!!

Let me know if the 318 can spin up to 4000 with no load.

After that you (we) are going to do a sea trial OK ??

FWIW, Bill D

 

[Toolate]

Thanks BillD- I may want to wait until the last day of the season to try that because I am almost sure its going to throw a rod through the block!

All great points- I will get the photo tach and make sure I am at least recording what the boat does accurately and try some at the dock revving too and get back to you.

The two boat examples I listed above are both running their props at cruising speed at about 1200-1250 rpm and using Larry's formula, are running at about 70% (gas) and 80% (diesel with DCQ 4 blade) propeller efficiency FWIW. I think the biggest thing that I dont understand is how to figure out how much load will be on an engine (total lack of experience) at a given rpm and gear reduction. I can make an educated guess based on other examples.

I could really use, as you say, another running example or 2 to add to the knowns though. I really enjoy this in case anyone cant tell.

 

[BillD]

Thanks BillD- I may want to wait until the last day of the season to try that because I am almost sure its going to throw a rod through the block!

All great points- I will get the photo tach and make sure I am at least recording what the boat does accurately and try some at the dock revving too and get back to you.

The two boat examples I listed above are both running their props at cruising speed at about 1200-1250 rpm and using Larry's formula, are running at about 70% (gas) and 80% (diesel with DCQ 4 blade) propeller efficiency FWIW. I think the biggest thing that I dont understand is how to figure out how much load will be on an engine (total lack of experience) at a given rpm and gear reduction. I can make an educated guess based on other examples.

I could really use, as you say, another running example or 2 to add to the knowns though. I really enjoy this in case anyone cant tell.

OK,
there is a lack of examples with 30 Sisu's and each boat's weight is different.

Use your own boat.

Get the photo tach and we'll do a sea trial. All the data you gather from the sea trial and knowing the pitch when the prop comes off will give you what you need for info to plan for next year.
Important: when you do the sea trial load the boat as HEAVY as you'll ever run it next year. Round up 8-10 adults with all their gear !..you'll need an extra set of hands for driving and running using the photo tach.

1. Accurate cruise rpms. The 318 you have now can get up to 3400 correct?
That's fine, get your speeds @ 3000 rpm and 3400 rpm.
2. You know the gear ratio.
3. When you haul the boat and take off the prop the prop shop will measure the diameter and pitch.

4. With these 3 pieces of data a couple of prop shops can get you very close to what speeds, load ect you'll have with the new power and gear.

Estimates of med/high cruise speeds will be very accurate, WOT will be a close estimate.

Best I can do for you on this.

 

[tunaorlater]

I have no experience with the 30 but with the 22 we got great results out of the acme prop co. A 140hp inboard gave us 18.5 knots wot and a 15 knot cruise. They did the prop calculations and when we didn't get full rpm on the first wheel it was shipped back and the second wheel was right on. No redoing the wheel, brand new in box. I dealt with bosler and sweezy in ny but any acme dealer will be good.

 

[Toolate]

I have no experience with the 30 but with the 22 we got great results out of the acme prop co. A 140hp inboard gave us 18.5 knots wot and a 15 knot cruise. They did the prop calculations and when we didn't get full rpm on the first wheel it was shipped back and the second wheel was right on. No redoing the wheel, brand new in box. I dealt with bosler and sweezy in ny but any acme dealer will be good.

This company? Our Propellers - Acme Propellers - The Most Accurate Way of Making Propellers

Name is from a cartoon but site is very sophisticated looking. Incredible performance from that 22 but I think the hulls are totally different sadly- that performance sounds good with 140hp. Interesting that their 20" props are available in 16- 26 pitch sizes. 20x26 is steep.

 

[BillD]

This company? Our Propellers - Acme Propellers - The Most Accurate Way of Making Propellers

Name is from a cartoon but site is very sophisticated looking. Incredible performance from that 22 but I think the hulls are totally different sadly- that performance sounds good with 140hp. Interesting that their 20" props are available in 16- 26 pitch sizes. 20x26 is steep.

Toolate,

Follow the steps I laid out for you.
You'll have all the "known" data any prop shop will need to come up with a 19 or 20 prop with the right pitch and a gear ratio.

Post your data here when you get the numbers.

I'll call Larry @ Accutech or Jim up @ Nautilus Marine or H&H in Salem.

Easy enough to do.

Trying to figure this out for your 30 Sisu using other length boats or hull designs will only get you a "guess"

 

[Toolate]

Hoping Mr Thompson will chime in and any other Sisu 30 owners. Could use some more back up. Prop shop is the next stop on the line.

Thanks again BIllD.

 

[BillD]

Hoping Mr Thompson will chime in and any other Sisu 30 owners. Could use some more back up. Prop shop is the next stop on the line.

Thanks again BIllD.

OK, offer stands.
Nothing else I can suggest to ya !
Good luck

 

[Toolate]

THink I missed something- Bill when you wrote I'll call the prop shops, is that what you meant? I read it as a typo- thought you were suggesting that I call them. Was I wrong? I would love it if you called them on my behalf if that is what you were offering!

I appreciate all of your help here and I still have lots to learn on this. Sorry for the confusion.

I am definitely ahead of myself Gurry. Not going to make any rash decisions before I have the boat out and the current prop measured (BillD's list) but I am definitely over thinking this and trying a little too hard to understand something I know nothing about unfortunately.

 

[captainlarry84]

Current engine is 318 Chrysler gas rated at 235hp at WOT that I think is supposed to be around 3700 but the engine wont turn past 3400 for some reason (rpms just fall off, engine stumbles, not sure if its fuel/air/spark or what) so that is what I have.

Using stock tach so that could be totally wrong- great point. Where can I get a good photo tach for a reasonable price?

Gear ratio is 1.91:1 currently.

Prop is 19" no idea of pitch (you couldnt tell from the pic? haha)

Propeller has be released from jail after serving 30 years.

Looking forward to any help anyone is willing to offer. What is the reward or punishment for longest post ever?

Trying to get a handle on all of this. First off like Bill said you need to seatest the boat and photo tach. You also need to get your old power up to full performance. This will give you a base line for the new motor & figuring out the wheel & gear will just be some simple math. Also a 2.5 gear is a lot. Also with the 2:0 gear you have more room to dial in a propeller. 19" is about it for your hull, so I would stay with the 19" diameter and pitch for the load. Assuming that you get your boat up to par. You will be increasing your HP by 37%. Therefore the new wheel will be 37 % larger that what you have. Meaning if you are turning a 19 X 17 your new wheel will be a 19 X 23. Based on the fact that you are only getting 3400 RPMs you could be very over wheeled or your motor is not up to grade. Your current 318 @ 235 HP should turn up to 4800 RPMs.

 

[BillD]

THink I missed something- Bill when you wrote I'll call the prop shops, is that what you meant? I read it as a typo- thought you were suggesting that I call them. Was I wrong? I would love it if you called them on my behalf if that is what you were offering!

I appreciate all of your help here and I still have lots to learn on this. Sorry for the confusion.

I am definitely ahead of myself Gurry. Not going to make any rash decisions before I have the boat out and the current prop measured (BillD's list) but I am definitely over thinking this and trying a little too hard to understand something I know nothing about unfortunately.

You get the accurate numbers, post them here, I'll make the call to a couple of prop shops as others would and will probably do.
Capt. Larry will be very close with prop, gear and performance with the engine you pick. His math is pretty good.
I bet a few others will be too.

This is up to you now !!!

 

[Toolate]

Will do- thanks for your wisdom again Larry and BillD. I will call a couple prop shops Bill (started this process already and waiting to hear) so please dont go our of your way. You have enough to do!

Larry, I thought the 2.5 was quite a bit as well but the only two boats I know of are spinning their props between 1200 and 1300 (propeller) rpms as far as I can tell. I was under the impression that 1500 or so is sort of the sweet spot for prop efficiency in general. Thanks for the thoughts on pitch too.

Current engine is definitely not running right and short of a can of carb cleaner I am not sure I am going to wrench on it but will see what I can do.

 

[dthompson004]

Please feel free to tell me I'm a jerk but its a gas engine. I have been doing some homework around here on this subject and there is an incredible wealth of info on this site if you search long enough (thank you to all who contribute, again . I would also like to preface this by saying that I will be working with a prop shop to sort this out and also that I understand that it is not as much a science as much as it is something that is learned by experience and through trial and error.

So, the engine will probably be a crusader 383 rated at 375 hp and max rpm is actually 5000. I would like to cruise at 3000 to about 3300 max just to keep the noise down if possible. I think the engine is making about 380lb-ft of torque and 250 hp at this rpm. Somewhere in there anyway.

Also think the gear will be 2.57:1 so prop rpm will be between 1200 and 1300 rpm at this engine rpm which seems slow to me. There is a 2.1:1 reduction gear available as well which would speed the prop up a bit- 1500-1650 prop rpm- so please tell me if you think I should go one way or another. Hoping to cruise 16kts minimum and be under 10gph.

Boat currently has a 19" prop but I don't know the pitch (it wasn't marked anywhere) and there is not room to go much larger in my opinion- maybe another inch or 2 at the very most. Right now, she will make 9kts at 2500rpm with 1.91:1 reduction so prop speed is 1308 and 11.7kts at 3000 so prop speed is 1570 and 13.2 at 3250 so prop rpm is 1700. If I project this out to 16kts, it means I would have to spin the current prop at about 2200 rpm which would mean 4620 engine rpm with the 2.1:1 gear and 5650 engine rpm with the 2.57:1 gear (too high).

Based on the info above, I will need a prop with more pitch to achieve 16kts if I go with the 2.1 reduction since the boat goes about 11.5kts at about 1500 prop rpm now and the 2.1 reduction would give me about 1500 prop rpm at 3000 engine rpm. Not sure how much or even if this is the way to look at it? Trying to gain more than 2kts by adding pitch seems like too much to me for some reason. Seems like I will need to spin a prop faster to achieve the speeds I want which either means higher rpms which I would like to avoid or a higher (numerically lower) gear.

Captainlarry84 offered this info in a post about a month ago- I saved it:

"Efficiency: The efficiencyof the propeller is what we strive for. 100% efficiency means that as stated above 1 of pitch per revolution equals 1 of forward movement. However 100 % is nearly impossible to achieve. It can however be improved if all of the above factors are carefully planned during your propeller selection.
In order to see just how efficient your current propeller is doing, here is a simple formula which will give you a ball park figure of efficiency loss do to slippage.

RPMs divided by Reduction Gear Ratio X=s Pitch X=s 60
(minutes) divided by 12 (inches per foot) divided by
5280 (feet in a mile) ==s MPH Divided by 1.15 =s Knots

Example:
Test vessel has a maximum RPM of 3000 revolutions it has a 2:0 reduction gear the propeller is a 19 X 19 size and the vessel does 20 knots. Based on this information we can compute the boats propeller efficiency with no other information needed.

3000 RPMs divided by 2 ==s 1500 X 19 ==s 28500 X=s 60 ==s 1710000 divided by 12 ==s 142500 divided by 5280 ==s 26.98 divided by 1.15 = 23.46 Knots

Therefore the above vessel should go 23.46 knots with 100 % propeller efficiency.
The boat however only goes 20 knots. Next we divide 20 into 23.46 =s .86.
20 knots is 86% of 23.46 knots therefore the efficiency of this boats propeller is
86 %."

Great info Larry- on my boat, if I use the 5000 rpm max for this engine, 2.57 gear and a 19x19 prop, is theoretically going to run at 30 knots (!). If I switch to the 2.1:1 gear the theoretical speed is 37kts (!, !). If I use 75% efficiency for my fat rudder and thick bottom paint these numbers leave me at 22kts/28kts at WOT for the two transmissions respectively. To my inexperienced eye, I would go with a 2.57 gear and a prop with a little less pitch so maybe a 19x18 or 19x17 to get the most power and torque out of the gear/engine/prop combination and avoid overloading the engine. On the other hand, this engine will have serious balls so maybe just leave it square and see what happens.

There is a member (Dthompson004) here with the same engine I am thinking of with 2.57:1 reduction and a 20x19 wheel and he as stated that he gets 14kts at 3200 and 21kts at 4100. This seems like a huge range to me (7kt increase in 900 rpm) and I wonder if he is overpropped because the boat is so fast at only 4100 on an engine rated for 5000. This would put him at around 25-27kts WOT which seems scary fast for this old boat. Mr. Thompson, have you run her up to WOT yet? Think we might both learn a little something from the results of that if you would.

Trying to make sense of Larry's math and Dthompsons real world example- there are close if you consider the relatively poor efficiency that I am sure these hulls deliver.

My final thought is that I may have an outdated view of gas engines because they are meant to spin faster today than they were in the past- its just that I don't want to listen to it go any faster if I don't have to. Do I have to? How low can I gear this engine at 3200 and not overload it and still get my 16kt cruise?

Told you it was a long post.:|

toolate, numbers are real, the boat does pick up a lot of speed in a short span, HP and torque is not linear either, might want to take a look at the HP performance curves of the engine. You can hear a huge difference particularly as the engine is looking for air, one of the first offseason projects is to get more air to the engine. Fuel useage shoots way up as well. (I know it is not a direct comparison but if you are interested look at the Tiara yacht website, they have some GPH and performance numbers on a 32 foot with twin Crusaders). I have not run the boat to WOT, previous owner did state that it would achieve 28kts. I don't have any inention to re-wheel, boat seems very efficient at lower RPM.