Donzi Ragazza prop help, no wot

[leakyz]

So I have been all over this website using the search function and reading but i cant find the answers i need so, I figure ill just ask and see what everyone/anyone thinks. I will spare you all the whole story about the truck loads of problems that have to be sorted out after you buy what was a "turn key boat"

So its a 1989 donzi ragazza bowrider
It has a 3 year old mercruiser 4.3 (that was purchased new)
The omc cobra drive and Ive determined the final gear ratio to be approx 1.6-1.7

the boat has a 4 blade alumin 15x16 prop now and with 2 people on board I am seeing a max rpm of about 3800 and with 4 people about 3500. I thought this prop would easily allow me 4500-4800 but no way. Any help,advice, critisism, anything is appreciated

guy also gave me a 3 blade stainless prop 14 1/2x19 I assume that would be worse though..

Thanks in advance

 

[steelespike]

Re: Donzi Ragazza prop help, no wot

Give us more info on the boat,length,weight,Deep V? deadrise, speed at wot rpm.
rated hp. Bottom clean and smooth?

 

[leakyz]

Re: Donzi Ragazza prop help, no wot

1989 donzi ragazza 19 ft
Mercruiser 4.3l I don't know the hp
Boat bottom is clean and smooth
I'm seeing 40mph at 3800rpm
Boat is 2800lbs prob 3150lbs as tried lately
I don't know the deadrise either, sorry

 

[steelespike]

Re: Donzi Ragazza prop help, no wot

You surely have some sort of issue or issues.I think most likely you should be running about
a 19-21" prop to about 4600 to 4800 rpm. approaching at least 50 mph.
Doing some very basic math disregarding slip and drag I don't think its possible to make 40 at 3800 rpm with a 16" pitch.So your tach might be reading a little low as well.
I would suspect some sort of motor problem.Do a compression check,Be sure it is in excellent tune. Timing right and advancing as it should.Throttle opening all the way.

 

[leakyz]

Re: Donzi Ragazza prop help, no wot

Thanks for the help again I will be sure to check the timing tonight when i get home and Im sure the throttle plates are opening, that was the first thing I checked. As for the tach, I will need to figure that out. Thanks again for your time. I will post back in a day or two with an update.