OUTRAGE 18 with 90-HP

Optimizing the performance of Boston Whaler boats
SpongeBob
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Joined: Tue Aug 09, 2016 5:42 pm

OUTRAGE 18 with 90-HP

Postby SpongeBob » Tue Dec 05, 2023 9:25 pm

I have a chance to buy an OUTRAGE 18 without an engine. I would plan to rig it with a 90-HP engine. I also own a MAKO 171 with a 90-HP engine whose hull weight is the same as an OUTRAGE 18. (See details below.)

Q1: in the history of Boston Whaler OUTRAGE 18 boats, has one ever been rigged with a single 90-HP engine?

Q2: how will an OUTRAGE 18 perform if powered by a single 90-HP engine?

Q3: will the performance of an OUTRAGE 18 with 90-HP be the same as the performance of the MAKO 171 with 90-HP?

ABOUT MY BACKGROUND
I know what the minimum horsepower is for an 18 Outrage. [In case readers do not recall this figure, it is 75-HP.]

I also know what the maximum horsepower is for an 18 Outrage. [In case readers do not recall this figure, it is 150-HP.]

I have owned an OUTRAGE 22 with twin 120-HP engine. I understand the concept of [rigging a Boston Whaler boat with engines that are at the hull's specified] maximum horsepower.

DESIRED PERFORMANCE
I’m not looking for blinding speed. I just want adequate performance to get on plane. I’m not going offshore 100 miles. I’m not pulling kids around on water toys. I just want to putt around the lake and the bay and be able to get home on plane.

CURRENT BOATS
I have a classic 13-footer powered by a Yamaha 30-HP two-stroke-power-cycle engine, which many might say is under powered. It suits my needs fine.

I also have a 1988 MAKO 171 with a Yamaha 90-HP two-stroke-power-cycle engine. This MAKO 171 and the OUTRAGE 18 both have a listed weight of 1,250-lbs. The MAKO 171 with 90-HP performs fine for my needs, and I have no complaints. I’ve even pulled water skiers.

I have a chance to acquire an OUTRAGE 18 with no engine for a reasonable price.

jimh
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Re: OUTRAGE 18 with 90-HP

Postby jimh » Wed Dec 06, 2023 9:59 am

SpongeBob wrote:Q1: in the history of Boston Whaler OUTRAGE 18 boats, has one ever been rigged with a single 90-HP engine?
Given the popularity of that hull, I would expect that someone, somewhere, at some time must have run the boat with just a single 90-HP. For example, in the case of the famous GAMBLER boat with twin 90-HP, I am sure it would have been tested to see if it could reach plane and maintain plane with a single engine.

SpongeBob wrote:Q2: how will an OUTRAGE 18 perform if powered by a single 90-HP engine?
Since 90-HP is more than the minimum power needed to get the boat on plane, I would expect an OUTRAGE 18 to get on plane with 90-HP. Exactly what the top boat speed will be can be estimated. I use the Crouch method, employ my Crouch's Calculator, and give an example below.

SpongeBob wrote:Q3: will the performance of an OUTRAGE 18 with 90-HP be the same as the performance of the MAKO 171 with 90-HP?
If the boat weights are the same, and the engine power is the same, the remaining influence on performance is the hull design. You will need someone familiar with the MAKO 171 to be able to make a good comparison. But in a general estimation, the deeper the vee-hull, the more power needed to accelerate onto plane and maintain plane. Compare the hull shape on the two boats. If they have a roughly similar amount of vee-hull angle, then you could reasonably expect that they would both accelerate onto plane at about the same speed and same time, given they weight the same and have the same engine power.

CROUCH METHOD TO ESTIMATE BOAT SPEED WITH 90-HP FOR OUTRAGE 18

See the calculator at

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

For the classic Boston Whaler moderate vee-hull design, the hull factor for use with the Crouch method is well known from many examples to be around 180. The engine power is known: 90-HP. The only remaining element is the boat weight. A rough estimate of the weight of a rigged boat is double the dry hull weight. Using the dry hull weight specified for an OUTRAGE 18 hull as 1,250-lbs, we can now use the calculator to predict performance with these inputs:

POWER = 90
WEIGHT = 2500
HULL FACTOR = 180

The calculated value for boat speed is then
(PREDICTED SPEED) MPH = 34

Even if the boat were more heavily loaded, the boat speed would certainly be above the hull planing speed. For example:

POWER = 90
WEIGHT = 3000
HULL FACTOR = 180
(PREDICTED SPEED) MPH = 31

If you eventually buy the OUTRAGE 18 boat, and if you eventually rig the boat wtih a 90-HP engine, then you eventually will need to select the propeller to be appropriate for that combination. Because the hull will be able to reach plane at around 25-MPH, to meet your criterion of being able to "putt around" and be able "to get home on plane", I am certain your desired performance could be achieved with proper propeller pitch selection.

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Phil T
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Re: OUTRAGE 18 with 90-HP

Postby Phil T » Thu Dec 07, 2023 11:11 am

While I do not recall anyone running a 90-HP engine on an OUTRAGE 18, I would not dissuade you from doing that.

You should take extra care to mount the outboard engine as high as practical and use a quality four-bladed stainless propeller.
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