60 hp on a bullet???

Outboard designs up to 14'

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60 hp on a bullet???

Post by TAB » Wed Aug 10, 2016 2:29 pm

so I have a mid 90s oil injected 60 hp merc with very low hours on it. I was wondering if it would be too much for a bullet.

I am wondering if the weight will be too much, its ~215#.

I understand that its only rated for a 40, but lets just say I have been known to put bigger out boards on boats :mrgreen:

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Re: 60 hp on a bullet???

Post by galamb » Wed Aug 10, 2016 5:54 pm

If you build the transom strong enough and balanced the hull properly you could hang a 350 horse off the back. And it's not the weight on the transom that's the problem (most incorrectly think the problem would be with the 200'ish pounds hangin' there) but it's the "thrust" the greater horses put on the transom.

When a motor is in forward gear essentially the prop, hanging down below the hull, is trying to pull the motor "under the boat" towards the bow. Because it's fixed by a bracket to the transom that "forward thrust" will cause one of two things to occur. If the transom is strong enough that "thrust" that is torquing the bottom of the motor forward and the top of the motor rearward will be prevented from moving and it will push the entire boat forward. If the transom is not strong enough, the top of it, along with the motor will be ripped from the back of the boat.

While there is no direct equation for how many pounds of thrust are produced by a single horsepower (thrust does not need forward motion to be measured, while horsepower does) it comes out roughly to 26 to 28 pound of thrust produced by 1 horsepower. So your 60 horse motor could impart upwards of 1680 pounds of thrust on the transom (see, that 200 pound motor is not an issue until you fire it up). In comparison, a 40 horse motor would only impart 1120 pounds of thrust on the transom. So your first order of business would be to built the transom, minimally, 50% stronger than original spec just to maintain the same safe margins.

Then the question would be, would you be able to control the hull?

Horsepower ratings are actually a mathematical formula that takes into account the length/width of the hull, transom width and height and the hull shape. If you exceed that (number) you could end up with an unstable hull once you exceed the hulls "max speed".

Think of one of those hydrofoils zipping down a canal and catching a little ripple and then going into a cartwheel. That happens when the hull is planing to the point where there is just not enough of it in contact with the water to control the craft.

Most hard chined, planing hulls have a max speed somewhere in the high 30 mph range. Beyond that they start to become "unstable" (not enough hull in contact with the water while underway). Before you say "bass boats can make 60 or 70 mph just fine", yes they can, but they are not "hard chined" planing hulls - they have been engineered to keep enough hull in contact with the water at that speed to maintain stability.

For all my babling, before hangin' a 60 on the back of a (max) 12' boat designed for 40 horse (and that may have been 40 BHP which would be a modern 35 horse, now measured in SHP, I think I would consult a marine engineer because I have no desire to do a cartwheel along the canal :)

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Re: 60 hp on a bullet???

Post by hoodman » Thu Aug 11, 2016 5:59 am

Have you already started the boat? Why not pick a design that is rated for 60 HP?

Building a Geronimo......!

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Re: 60 hp on a bullet???

Post by Moeregaard » Thu Aug 11, 2016 6:04 am

hoodman wrote:Have you already started the boat? Why not pick a design that is rated for 60 HP?
The Rebel comes to my mind.

A boat is just a wooden box with no right angles.

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Re: 60 hp on a bullet???

Post by TAB » Thu Aug 11, 2016 11:06 am

I want something rather small, and fun. I would like to be able to run the "mini" parker endure next year. so under 12'

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Re: 60 hp on a bullet???

Post by BillW » Thu Aug 11, 2016 5:35 pm

If you increase the horsepower by 50% (from 40 hp to 60 hp) then you can expect the speed to increase around 17%. That assumes the motor set-up and propeller are good.

IF the 40 hp will push a given boat to 32 mph, then a 60 hp will push it to about 38 mph, all else being equal. This is not a guess on my part, but factual
info from the Quicksilver (Mercury) Propeller Slip Calculator. That's an example, the not actual speed of a Bullet.

This little slide-rule (Mercury part number C-90-86147) is very handy. Especially if you have to prove to your brother-in-law that his boat cannot
possibly be as fast as he claims.

I don't know if the calculator is still available.

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