rebedro

I have a 1989 Donzi 25ft cubby cabin. I was running a 260 hp omc in it. My prop set up is 14 1/2 by 19 inch pitch 3 blade. Now the thing is i just installed a 325hp jasper engine. the WOT on this new engine is 4900 rpms. Im still using the old prop from the 260. I know i have to upgrade to a new prop So could the smart guys out there help me?. I want to plane faster at low speeds and i want to reach the full RPMS for the boat. Right now im only getting 4000 i want to know how to get the other 900 rpms.

Griff

What were the WOT rpms on the old 260hp engine????

rebedro

it was about 3000rpms not much more.

Powerquest_Baby!!

Something isnt right here....You would need a 15p prop to get your RPM's right. Have you weighed the boat? What ratio is your drive? IF your drive is 1.5 then either your HP is wayyyy off or your boat weighs 3 tons.

tinman565

Seems to me that a 25' Donzi cuddy with 325 hp should love that 19p 3 blade prop. Just guessing tho.

rebedro

here is the thing im wondering why am i only getting 4000rpms? if the WOT is 4900rpm. The boat runs good but i would think after adding 65 more hp that i would need a different prop. Dont you guys think so? I do turtle trips and fishing trips with the boat. Its always loaded with people I need a prop that can get me on plane with the load im carring plus planes at slower speeds. HELP ME GUYS I want my 900rpms that im missing.

ImaPoser

To get that 900 rpms, you need to go from a 19p to a 15p. But, if you added that kind of HP, unless it was barely able to get on plane before, something else is wrong. If it was propped right before, you should be banging the rev limiter with ease now. For that size boat with that HP, a 19 sounds like it should be the right size, so I"d be looking at what else is going on.

Griff

That is extremely low and shows that the prop was way too big before.

The WOT rpm is controlled by the size of the prop that you are trying to spin. You need to drop down in pitch. I suspect that drive gear ratio is way too high.

rebedro

what could be off when this engine is 6 month old its new. with the old engine it took everything to get the boat on plane. so i would have to flat line it to get on plane. Now if im carring people then its really hard to get on plane. do you think i need a 4 blade or something else i will try a 15p and see what happens.

Griff

You are for sure overpropped if the engine is tuned correctly. A smaller prop will get you on plane much easier as well.

Like I said, your outdrive gear ratio is probably holding you back.

For example, I have a 1.36 outdrive gear ratio and spin 24" props to 5000rpms. If I had a 1.50 ratio, I would be spinning 27" props.