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new owner looking for help

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jeremymik84

New member
just bout a 96 caravelle 1750 se fish and ski boat with mercruise 3.0 and alpha one.. at 4200-4400 mph im only seeing about 30-32mph (although not fully trimmed up) the prop has a little nick on it and im going to replace it. it has a 19" x 14 1/2 3 blade prop on it now..im looking to get more top speed and get the RPM right at WOT.. i havent tried WOT yet because at 4400rpm its nowhere near WOT so im thinking its underpropped... any suggestions would be awesome...im new so im still learning

sometimes i can get it up to about 35 while trimming up but i still havent tried at full trim up. im scared to go WOT because i dont want to over rev it and cause damage.. just trying to give as much info as i can



 
At wide open throttle, 1/2 tank of gas, one 200 lb person on calm water or very little waves

you should reach max rpms of 4800 (max) at wide open throttle and with the drive trimmed out just before cavitation occures (cavitation is when you trim it up just a bit to far the boat looses speed due to too mch air traveling over the prop) (prop to high or out of water).

If you are reaching the max rpms to soon then the pitch is incorrect. In your case to small........

So a 21 or 23 pitch will give you higher speed. The key is not being able to go over 4800 rpms no matter what.

There is a sacrifice that occurs. Due to this bieng a fish/ski it would appear it is propped for pulling a skier. Better hole shot vs top end.

So the higher the picth the worse the hole shot but higher top speed in MPH. There will be a point where this stops based on the motor/boat design so it is not a endless gain........
 
okay ill try that out asap. hopefully this weekend. ill have to put my fear aside and go full throttle and see what happens.. i dont pull any skiers and doubt i ever will.. if i do i can keep the prop i have now as it only as a small ding in one of the three fins. i do however pull a tube occasionally so i would stilll like to effectively do that but all the while gaining a little bit more top end. if i jump to a 21 pitch will it kill me being able to pull a tube with one person on it? i normally ride with 2 adults ( im 165 my girl is 115) and 2 (sometimes 3) kids under the age of 10. if i jump to a 21 pitch prop, im having a hard time finding 14 1/2 diameter which is on it... what are the effects of going larger or smaller? which would you recommend for what im trying to accomplish
 
Each change in pitch gives you ~200-300 rpm change.

Do not over rev your engine.... This is not a race engine.
If you go past 4800 do not do it for very long. If you reach 5200 and still
have more throttle then stop there.
If you reach ~5200 and have no more the go to a 21 pitch.
If you have more then go to a 23 pitch..
 
okay will keep that in mind... what about the diameter part? its a 14 1/2 now, what difference would it cause to go to a 14 or 14 3/4 or 14 5/8?
 
jeremymik84 said:
okay will keep that in mind... what about the diameter part? its a 14 1/2 now, what difference would it cause to go to a 14 or 14 3/4 or 14 5/8?
Click to expand...

Ayuh,... Blade area, pitch, basic design, rake of the blades, 'n amount of cuppin',...

Generally, pitch goes Up, diameter goes Down,...
 
Until you get the boat to wot and trim up properly you wont know what you need to do about the prop. Don't be afraid to go to wot or to trim up as high as needed. Boats are not cars and they are designed to be operated at wot an, in fact, must be to determine the correct prop for a given application. And as far as trimming the outdrive goes, you boat will let you know when it's too far up. Your boat will either start to porpoise or the prop will start sucking air. Neither are catastrophic and are easily corrected by trimming back down slightly until the behavior stops.
If you are only getting 32 mph at 4400 rpm then my first thought is you are not trimmed efficently. My boat, different engine so yours may not be exactly the same, goes roughly 30mph at 3000rpm when trimmed for the load.
My suggestion to you is to play with the trim until you see how the boat and engine responds. Set the throttle at a base rpm, just pick a number, say 3000rpm. Then play with the trim, bump it up in short bursts and watch how the boats attitude, speed and rpm react. Your goal is to trim as high as possible without making the boat behave badly, like porpoising or the prop coming out. As a plus, an efficiently trimmed boat will get better fuel economy. Once you gain some confidence with using the trim then try the same throttle and trim settings but rearrange your passsengers and see how that affects your boats trim. You should notice a difference in hoh the boat rides front to back and side to side.
On the prop, one nick should not affect speed that badly. Id guess that a 14.5 x 19p is nearly the perfect prop for your boat. Maybe going to a 21 would get you a couple mph but at the expense of hole shot/quickness to plane. If you decide to change props, after testing at wot, idngo with a 14" or 14.5" to avoid possible interference issues.
Good luck.
 
sounds good guys, like i said ive played with the trim a little, ive trimmed it up a bit and gotten up to about 35 but that was at 4000-4200 rpm.. if the water is choppy at all it will start porpoising, not terribly, but it does.. when its calm and i can get on her good ill get back with you guys and let you know. as for now thank you for your help. i went out this past monday and had the trim up a bit and it didnt porpoise at all until i ran into another boats wake so i trimmed her down.. but the speed is very inconsistent.. i dunno if its the speedo or tach but sometimes at 4000rpm it will go a diff speed than other times at 4000rpm
 
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