#### Toolate

##### Admiral

- Joined
- Aug 6, 2013

- Posts
- 6,620

- Likes
- 2,542

- Location
- Southwestern CT

- First Name
- Ben

- Boat Make
- 1998 36' Sabre Express CAT 3116TA's, 2015 ROS Scotia 16 Etec 50

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.:|