1988 25-foot Hull with Whaler Drive Re-power

Optimizing the performance of Boston Whaler boats
mike
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1988 25-foot Hull with Whaler Drive Re-power

Postby mike » Fri May 05, 2023 7:42 am

I have a 1988 25-foot hull with Whaler Drive. I plan to re-power with a used pair of 2001 Yamaha 250-HP Ox-66 engines.

Give me a recommendation for propellers.

ASIDE: the capacity plate says maximum power is 450-HP.

jimh
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Re: 1988 25-foot Hull with Whaler Drive Re-power

Postby jimh » Fri May 05, 2023 7:52 am

Give more information about the boat and the engines.

Q1: what is the gear ratio of the Ox-66 250-HP engine?

Q2: what is the maximum allowed engine speed of the Ox-66 250-HP engine?

Q3: what will be the total boat weight of the 25-foot hull when on the water with your regular load of passengers, fuel, and gear?

mike
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Joined: Wed Jan 20, 2016 7:34 am
Location: Florida's Space Coast
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Re: 1988 25-foot Hull with Whaler Drive Re-power

Postby mike » Fri May 05, 2023 8:01 am

Q1: what is the gear ratio of the Ox-66 250-HP engine?
1.75:1

Q2: what is the maximum allowed engine speed of the Ox-66 250-HP engine?
5500

Q3: what will be the total boat weight of the 25-foot hull when on the water with your regular load of passengers, fuel, and gear?
I'm estimating 5200 lbs

jimh
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Location: Michigan, Lower Peninsula
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Re: 1988 25-foot Hull with Whaler Drive Re-power

Postby jimh » Sat May 06, 2023 2:40 pm

With the new data supplied, I can make an informed estimate for a propeller.

First, I will calculate the anticipated top speed for the boat using a calculator I have created that is based on the method of naval architect George Crouch. The calculator estimates the speed of a moderate planing hull boat based on power, weight, and hull form. See

https://continuouswave.com/calculators/crouchCalc.php

I will use the power and weight given for the boat under discussion, and a hull factor of 180, based on my own experience with a classic OUTRAGE hull with a Whaler Drive. This is a conservative hull factor, and if the boat can be pushed to high speeds and raise most of the hull out of the water, the apparent hull factor could increase.

The inputs are thus
LBS = 5,500
HP =500
HULL FACTOR = 180
MPH = to be calculated

The algorithm calculates 55.8-MPH. Now using a target top speed of 55-MPH, I can compute a propeller pitch. I will use my own Propeller Calculator. See

https://continuouswave.com/calculators/propCalc.php

The inputs are thus
RPM = 5500
RATIO = 1.75
PITCH = to be calculated in INCHES
SLIP = 10 (this is an assumed value based on a propeller working properly and with an accurate pitch marking)
MPH = 55

The algorithm calculates a propeller pitch of 20.5-inches.

Rather than recommend a set of 20-inch pitch propellers, I would start with 19-pitch. With 19-pitch you should still have a 51-MPH boat, and I think you will have better acceleration under load. Getting too much pitch is hard on the engine, causing it to be bogged down by too much load, to run inefficiently, and possibly causing damage to the cylinders. The goal in pitch selection is to keep the engine happy, to let it be able to accelerate rapidly to its recommended maximum speed range, and to not overload the engine with propellers with too much pitch. Remember, these are 22-year-old engines, and you will probably want to keep them running for a while. Re-powering with new 250-HP engines will cost a fortune, and their weight may be too much for the Whaler Drive.

And 51-MPH is plenty fast for most sea conditions. If that last 3 to 4-MPH is really important, buy a pair of 20-pitch propellers and see how the engines run with that load.

As for other particulars, you do not mention if the engines are counter-rotation. Getting a left-hand propeller on a sea-trial basis may be harder than getting a pair two right-hand propellers if the engines are not a counter-rotation/standard-rotation pair.

On my Whaler Drive boat there is very little need for a bow-lifting propeller. I think a stern-lifting propeller is probably more appropriate with the Whaler Drive. Stern-lifting propellers DO NOT have highly raked blades. Some think a four-bladed propeller may give more stern lift. I can't verify that as I have not made scientific measurements, although I have a four-blade propeller on hand and I have run with it for many hours. I keep going back to the three-bladed propeller because fuel economy is better.