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Yet another prop question

S

switlikbob

Contributing Member
"Okay, so I just myself a shin

"Okay, so I just myself a shiny new motor that is rated at or about 300HP. I am using a 280 outdrive. I removed the prop that came with the boat, which was a 15x15. I need a starting point here for a new prop. I have looked at various prop calculators, and they all seem to be inconsistent with their results. My boat is a 1988 Marathon, weighing about 4500 - 4800lbs. The gear ratio of the outdrive is 1.61:1. Can anyone help me out please?"
 
Why not leave the 15x15 on the

Why not leave the 15x15 on there and see how it performs? It would seem easier to choose the proper prop if you knew the RMPs at WOT with that known data point.
 
"Well, that prop was removed i

"Well, that prop was removed in a quite violent fashion using torches, a diamond tip grinder, and a reciprocating saw. I was in 3 pieces by the time I got done with it. I had to remove it due to leaking seals. If I can find another prop of that size for a reasonable price, I will probably try it."
 
"Based on the bit of info you

"Based on the bit of info you provided I threw some numbers into my prop software and at a quick stab it looks like your 15 x 15 is the initial recommended prop for that setup. But like Bob stated, you should run it and see how it works before you invest any money in another prop that may or may not improve anything for you."
 
"Bob, my boat is a 22' Dou

"Bob, my boat is a 22' Double Eagle with a Chevy 350 (rated at 307 HP) and a 280 outdrive 1.61:1. The boat weighs about 6000 lbs, including all gear, fuel, water and two persons. I am running a 15x19 prop. So my guess is that your boat should be able to run a minimum of 15x17 or a 15x19, perhaps even a 15x21. You should conduct some trials before you can find out for sure. Try to borrow some used props from a propeller shop or from a VP dealer who has a boneyard.

A bit of advice: invest in a fuel flow meter (even if a cheap one) so that you can calculate the boat's fuel consumption per nautical mile with different propellers. The results will help in the selection. For example, my boat is slightly overpropped (on purpose), but I am obtaining a steady 2 liter per nautical mile fuel consumption between 3200 and 3900 RPM, and a boat speed between 24.5 and 30 knots. None of the other props I tried could match that performance."
 
Thanks for all of the great ad

Thanks for all of the great advice guys. I was kind of guessing that a 15x17 would do the trick for me. I need to find some cheap props to test with.
 
"Update on my situation:

I

"Update on my situation:

I got the boat in the water on Saturday. I am very pleased with the new engine. However, top end speed still seems to be an issue. The boat now has a 1978 chevy 350 with an aggressiv eclevite marine cam, 750 cfm carb and spacer as well as flat top pistons and heavy duty valve springs. The cam was supposed to get the motor to 300HP. I am kind of wondering if it is there. Anyway, I am running a 15x17 aluminum prop. The rpm's top out at around 3800-3900 rmp. Top speed seems to be about 31mph. Does this sound right?"
 
"hi bob, it sounds like you ar

"hi bob, it sounds like you are not getting 300hp, is it getting full throttle, i get 5000rpm with a 15x15 and 4300 with a 15x17 and i have only got 170hp in a 24ft glass cruiser."
 
You need to find out what the

You need to find out what the recommended WOT is for that motor. Your optimum WOT should be about 200-250 rpm below max...
 
"I agree that I need to find o

"I agree that I need to find out what my max rpm is on this new motor. However, I have no idea how to do that. Can anyone help? Here is the block casting number: 3970010. I am going to go ahead and assume that it is 4500rpm. It doesn't sound like the secondaries on the carb are opening up until I hit 3300rpm. I took her out last night and played around. The trim tabs do get me a few miles per hour when I fool with them. It seems that WOT it giving me no more than 3700rpm. At around 2500rpm, the boat goes 25mph. So how can an additional 1200 rpm only get 6 mph?"
 
"Update...I called the company

"Update...I called the company that built the motor for me. He told me that the max rpm is 4500. I am starting to think that there is possibly a problem with the carb, or the intake manifold is too small...the motor was stalling out on me right after I put it in gear and start to give it gas. Mayeb I am running a bit lean?"
 
"That's what the owner tol

"That's what the owner told me...4500rpm is the max. I seriously doubt that the motor is 300HP. He told me that he installed a clevite 300-320 hp cam. That means nothing to me really. I asked him if he had some kind of dyno sheet showing the engine's horsepower rating and he told me he did not. To answer your first question, my carb is not a holley, it's and edelbrock/weber carb, and I am 99% sure that it is a automobile carb. The motor is most likely a 1978-1980 model chevy 350."
 
"Bob, I hope my engine will la

"Bob, I hope my engine will last me a few years (touch wood
biggrin.gif
). But if in the future I had to replace it due to a catastrophic failure, I think I would consider something like this:

http://bpi.ebasicpower.com/engines/base/350.htm

Who did you get your engine from? You may want to make sure you got a proper marine version engine and not a truck or car engine. Also, I don't think you can get 300 HP at 4500 RPM from a 350 block.

Just in case you are interested, my 350 has a Melling performance camshaft number 22124."
 
"bob, he may have installed a

"bob, he may have installed a cam that is good for 300 hp, but what about the heads, comp ratio etc: it s the combination that makes the hp, not just the cam, i have put performance cams in stock engines before with no gain at all because the intake and exhaust flows were not high enough."
 
"El, I got my new longblobk fr

"El, I got my new longblobk from here:
http://www.rapidomarine.com/default.aspx?p=/engineparts.aspx
I like the e-basic engine, but I had all of the parts needed to build a long block up, so I used them ans saved a good bit of money. According to rapidomarine.com, the engine is a true marine engine. So far, i am very happy with it and it runs well except for a dead spot at initial acceleration. I think I can fix this with carb adjustments. Now that I think about it, the cam in ther emay be a melling, I can't remember. I do have the box that the cam came in at home. I'll have to check it out.

Brendan,
I agree with you. That's what I meant when I said that the 300HP cam means nothing to me. I do have a pretty nice intake manifold, a huge 750cfm carb & spacer. I'm not sure about intake / exhaust flows, but you can check out the website listed above if you want. tell me what you think. Thanks!"
 
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