Larry- I need some Prop and shaft advice?

Powderpro

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I had a few questions about prop size and shaft size recommendations and requirements. I'm seeking the advice of a pro.

Boat: New build, modern DE hull w/ keel, length at waterline 37', beam 13', approx weight 18,500 - 20,000 lbs.

Engine: JohnDeere 9 liter, 500hp at 2,400 rpms, 2:1 reduction.

Question 1. In a boat this size and weight, do you think I would see a performance increase (cruise and top speed) going to a 30" diameter prop vs. a 28" diameter prop? Boatdiesel's prop calculator says I can go with 2" additional pitch with the smaller prop, which would theoretically equal more speed; but can the more efficient 30" prop make up for 2" less pitch?

I know from first hand experience in a boat this size/weight, that a 28" prop will give better performance than a 26" prop, but at some point it would seem you would get diminishing returns and a bigger prop would just be overkill. So I'm wondering if a 30" prop on this size of boat would be overkill and deliver no better performace than the 28" prop. In my experience, the larger prop always delivers better performance, but I've never used larger than a 28". What has been your experience or what is your opinion? Cost not being an issue, what diameter would deliver the best performance at cruise and top speed?

Question 2. I'm under the understanding that a 2" aquamet 22 shaft is fine and adequate up to 500-550hp and 28" diameter prop. But it would be wise if going up to a 30" diameter prop with the same 500-550hp, to jump up to 2.25" shaft. Is my understanding correct? And if the 30" prop required a 2.25" shaft, is aquamet 19 enough or should it be aquamet 22?

Question 3. Is a 2.25" aquamet 19 shaft stronger than a 2" aquamet 22 shaft?

Thanks for your advice,

Brian
 

jerseysportfisher

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Brian,

If i were ordering it. I would want the biggest wheel i can get on it without getting into the boundary layer. being a 30 inch wheel the slower it goes the more efficient, i would want a 2.5:1 trans.

as for comparison of shafts, its a hard compromise, being they are different alloy, exerting different properties, each has its pro's and con's.

19's a little softer absorb a little more and are capable of twisting abit.

22, are a bit stronger but more brittle being a harder alloy.


good luck, POST PICS !!!
 

jerseysportfisher

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oh yeah, i would put the bigger shaft in, just for the reason when it comes time to repower, and your bored of 500hp and want more, no major surgery
 

pointblank1

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Hey Brian.

What kind of luck have you had with the boat diesel prop calculator? And what are you using for "planning type" entry. I am trying to prop my build as well and have used that same calculator several times and am just a little leery of it.
 

hntrss

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I have a friend with twin 900 mans and 2 inch shafts, i feel like everyone jumps up in size to easily. His shafts are splined as opposed to keyed, but he has never broke one. Wouldnt the smaller bore increase efficiency/ speed?
 

Powderpro

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I have a friend with twin 900 mans and 2 inch shafts, i feel like everyone jumps up in size to easily. His shafts are splined as opposed to keyed, but he has never broke one. Wouldnt the smaller bore increase efficiency/ speed?

2" shaft connected to a 900hp Man? That sounds really small for the HP, what size props?

Tuna- It's on a 38' Lowell Brothers, a 30" prop would be real tight keeping a 4" prop tip clearance as the apeture is a touch over 34". I was thinking I could shave the skeg down about 1"- 1.5", and add an aluminum or stainless skeg shoe around the fiberglass skeg like Wesmac does (see pic). That way my shoe would be thinner in height, giving me a perfect size apeture for a 30" prop, and would still be plenty strong with the added strength of the metal. It can be done easily enough, I'm just wondering if the bigger prop would give me a performance boost. I know once I have some weight on the boat, the bigger prop would definitely shine, but honestly a 28" prop is quite efficient, so I know I would be happy with its performance as well.

I was thinking of adding a metal skeg shoe anyway, as the river I run in and out of is filled with sandbars and rubbing bottom is a common thing. Bigger is better as they say;).

106.jpg
 

Powderpro

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Hey Brian.

What kind of luck have you had with the boat diesel prop calculator? And what are you using for "planning type" entry. I am trying to prop my build as well and have used that same calculator several times and am just a little leery of it.

The boatdiesel calculator has always been real close in all the boats I have built. I usually over-estimate a little bit on the weight when I input the numbers into the calculator, so my actual top speeds are a little faster than what the calculator says it will be. My experience is the calculator will get you real close to what you need as long as you are putting in the correct information.

With this next boat I will be starting on in August, the calculator is calling for a 28x29 prop. If I move up to a 30" prop, it is calling for about 30x27. That is using an "average planing" hull. My experience has been that a good DE hull is very close to an "average planing" hull, so I would take 1" of pitch off the calculator's suggestion, and probably be spot on. Another thing is I will be using a DQX, which has bigger ears, which will also tend to reduce the pitch I would need. So it's looking like a 28x28 or 30x26 in a DQX 4 blade prop.
 
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tunaorlater

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I personally would go with the 28 because it would be smoother and allow for more water to pass between the tip and hull bottom but I am not loading my boats up like you are. That's a great shot! Any updates?
 

Powderpro

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Just to be clear, that pic of the Wesmac skeg is not my boat, I was just posting it to show how I could reinforce things. I believe that was a pic of a 38 Wesmac that Stewart Workman finished a few years back. Wesmac and Stewart Workman both do really nice work.
 

F/V First Team

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Supposedly 2" shafts are the "strongest" inherently
Remember they're trying to sell shafting.

I wouldn't go above 2" in my own boat personally, wouldn't matter what I was running for an engine. I'd put it behind one of the Packards without a moment of hesitation.

Actually been thinking of doing just that with my 1.75"...
 

Powderpro

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He not going to keep that one long enough to even think about repowering :D Right Brian old buddy ???

Steve you got me, you are correct, I can't imagine keeping a boat long enough to need a repower. However, my dad always says this is his last boat, and then he agrees to build just one more... so any day now it may really be his last boat. And trying to finish a boat by myself in the short amount of time between seasons would be a monumental task that I'm really not interested in. So you never know, the next one may be the last;).

The owner of the machine shop (who has been in business for 35 yrs) where I get my running gear thinks a 30" prop on a 2" shaft is living on the edge. He knows of guys who have done it, but he recommends 2.25" if going to a 30" prop. That's just one man's opinion, and he is probably being conservative so nothing comes back to bite him. But I was looking for more opinions on the subject from other professionals.
 

steveinak

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Steve you got me, you are correct, I can't imagine keeping a boat long enough to need a repower. However, my dad always says this is his last boat, and then he agrees to build just one more... so any day now it may really be his last boat. And trying to finish a boat by myself in the short amount of time between seasons would be a monumental task that I'm really not interested in. So you never know, the next one may be the last;).

The owner of the machine shop (who has been in business for 35 yrs) where I get my running gear thinks a 30" prop on a 2" shaft is living on the edge. He knows of guys who have done it, but he recommends 2.25" if going to a 30" prop. That's just one man's opinion, and he is probably being conservative so nothing comes back to bite him. But I was looking for more opinions on the subject from other professionals.

Hey as long as you and Dad enjoy building them keep doing it, spending time with ones dad is always good, i still always think of all the times my dad took me fishing and he always stopped by the boat or the house when he was in my area. I miss ya Mike(RIP).

Put the bigger shaft in and be done with it !! That way if you run over a net you can just cleat off the lines and put'er in gear to chew them up and be on your way:eek:.
 

hntrss

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The boat with the mans is super fast, and has its second set of motors. Over 15000 hours on the boat and crazy high performance, high pitch racing style props. Not sure of the size, but over 30 in diameter for sure. Never broke one ( he has replaced them from wear) and wouldnt ever go bigger. There is really no need to go bigger, although splining is inherently stronger and less prone to breaking. I agree with Mr . Travis here . In my opinion the giant hub will steal too much performance from you if speed is your thing
 

jerseysportfisher

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The boat with the mans is super fast, and has its second set of motors. Over 15000 hours on the boat and crazy high performance, high pitch racing style props. Not sure of the size, but over 30 in diameter for sure. Never broke one ( he has replaced them from wear) and wouldnt ever go bigger. There is really no need to go bigger, although splining is inherently stronger and less prone to breaking. I agree with Mr . Travis here . In my opinion the giant hub will steal too much performance from you if speed is your thing


i'm willing to bet those shaft are not just standard aquamets, they are more then likely very expensive custom alloy shafts that cost a F load.
 

hntrss

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I am not sure if they are somthing special, but if they are , how special could they be? We are talking about a boat that crusies close to 40 knots, as opposed to boats that are trying for 18-22knots. Definitely not looking to argue, just maknig a point that bigger is probably not better or necessary (or worth the added expense for couplers, tubes cutlass bearings, zincs and propellors) Travis probably beats the shit out of his 1.75 more on the race circuit than we could ever hope too under normal fishing scenarios, right? Everyone here would no doubt say that 1.75 is totally undergunned for a high tech, high end, racing boat, no? Yet here the man is telling you that he has no problems.
So, again, I have no problem with bigger shafting, just saying powderpro may not need it. Possibly save a couple of $$ and go a little faster to boot!
 
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