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What difference would a long leg outboard motor on a boat designed for a short leg do
I am looking at boats like TNT
Dino

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Its not weather you win or lose its the party after

Basically, what it would do is put the thrust point 5 inches lower than necessary. This can cause a funky handling boat, especially when turning hard, it would try to roll more.

This also means you would stike the lower unit into the mud (rocks, Sand, add your local bottom type here) that much sooner.

Additional drag would be introduced, which would mean less fuel economy.

It's just smart to either build the boat to suit the motor, or buy a motor to suit the boat.

Best of luck with your project and keep us in the loop.

Brian

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Noah was a first-time boatbuilder

if you havent built the boat yet you just configure the transom to use the long shaft...This is not an uncommon modification

Thanks for the replies
I know to put the motor on what it designed for
The motor I have is a long leg and wanted to know if putting it on a design for a short keg was going to be to big a problem
I have done some other investigations since and found that the difference is 127mm
So my next question is to lengthen he transom 127mm would not affect the boats performance to greatly
Dino

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Its not weather you win or lose its the party after

it doesnt effect the performance of the boat at all. The boat doesnt know if it has a long or short shaft motor on it. As long as you adjust the transom to compensate for the longer shaft your good to go. I had to do this on my Rebel. Many people have hd to do it as most of the plans were designed for short shafts when they were more avialable but today short shafts are hard to get.

Thanks for the reply
I don’t know much about boat design and been reading the boating 101 and goes into the basics of boat design
They go into CB and weight distribution so i didn't want to up set the performance to much
I have done a lot of radio controlled airplane design and building I know just putting a different brand of motor can change the flying cathartics of a plane
Next question I weigh 110kg /124lb and 6’4”tall
On the TNT should I move the seating poison?
Dino

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Its not weather you win or lose its the party after

You can build the boat,then mock it up to figure out the seating position.

Once you get the steering in place & sit in it,you can figure the fore & aft & height to what feels best for you

_________________
Boat building can best be defined as an endless series of
tragedies obscured by the occasional miracle, followed by a good bottle of beer.

Don't Dream Your Life, Live Your Dream

Very wrong.
Using a jackplate and setting the 150 to run the cav plate even with the bottom of the keel will make a 10-12 knt (gps) difference from the stock height. When the difference is a 50 knot boat, or a 62 knt top... that is a major effect on performance. Every inch of the shaft introduces amazing amounts of drag. Just go hang a 2x8 into the water about 5" at 50, and check the effects for yourself. Essentially that is what your motor leg is.

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Some plan to stroll through Saint Peter's Gates, I plan to go through them at 150mph... backwards... in a screaming ball of flame, with a glimmer belt wrapped around my head, and a NOx button in my hand.

I was talking about changing the boat from short to long shaft. IF, as I said, you adjust the transom your good to go. With the transom adjusted the boat will not know the difference and it will NOT effect performance at all. I was not suggesting he run the long shaft on a transom designed for a short shaft that was not what I was saying...

caber,I believe the question was performance difference given a 20 3/4' motor and transom,or a 15 3/4 motor & transom.....the prop & cav would be in the same position.It would obviously be better to use the 15" on this application,but the increase in weight(powerhead) higher probably wouldn't be determinable.

_________________
Boat building can best be defined as an endless series of
tragedies obscured by the occasional miracle, followed by a good bottle of beer.

Don't Dream Your Life, Live Your Dream

Remember,we are talking an 11' boat with 25-35 hp maybe 50 at the most

_________________
Boat building can best be defined as an endless series of
tragedies obscured by the occasional miracle, followed by a good bottle of beer.

Don't Dream Your Life, Live Your Dream

I put a long shaft on my boat (Tuffy) and jacket the transom up to meet the 20" of shaft. Big problem, cost more, more time, more frustration. There are minimal shops around that cut your 20" into a 15" shaft but I would recommend just either swapping the shafts (20" to 15") or buying a different engine.

You can get 50's in 15" shafts you know. Tohatsu MD50 TLDI comes in a 15" shaft. That is the only new motor that I can think of that has a 15" shaft. Comes in a 40hp too.

25hp short shaft (SS) with tabs and one average driver does around 35mph I believe. Raptor; you can modify engines in the lower hp range, keeping the same weight but increase the hp by 40%. Thus increasing performance, but not exceeding the 200lbs range.

Some Yamaha's and Merc's can be modified drastically. You can gain up to 6hp with high performing heads with the 20/25hp Yamaha 2 stroke, and add better acceleration with carbon reeds. Solid engine mounts wouldn't better a bad idea either due to increased handling.

Mercury Racing created 25XS and 44XS outboards. The 25XS has a 12" shaft (as well as the 44XS). It has the same block as the 25hp Mercury 2 stroke but uses racing components such as better pistons, heads, lower unit, shaft length, and most of all important high grade gasoline. With a lower shaft length your center of gravity will decrease (good) thus putting you in better control at high speeds.

Also remember that whenever you see a reading of weight it is in the "dry weight" form. Wet weight includes: oils, gas, lubrication, etc.

I know I ramble on but having a monster on your hull is just something to think about.

As for your weight distribution problem. It all depends on your choice of motor, battery size, how much fuel you have. When in doubt spend the extra cash and get some Minn Kota or Lenco electric trim tabs.

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Tuffy with '75 Merc 650 SS.

Ah, that makes sense.
I don't know why, I had this vision of a long shaft hanging way down, digging up clams or something.
Spring always destroys my thought process. must be the shock of surviving another winter, or something.

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Some plan to stroll through Saint Peter's Gates, I plan to go through them at 150mph... backwards... in a screaming ball of flame, with a glimmer belt wrapped around my head, and a NOx button in my hand.

Or the beers over the winter waiting for the weather to break!!

_________________
Boat building can best be defined as an endless series of
tragedies obscured by the occasional miracle, followed by a good bottle of beer.

Don't Dream Your Life, Live Your Dream