long shaft transom height?

[Baldur]

Is there a standard transom height for a long shaft outboard, from top of motor mount to bottom of hull? I think it is around 20 inches or so but I would like to double check before I start cutting.
Meaure twice, cut twice !!! lol

[nevd]

Yes, longshaft transom is 20 inches, but it is vertical and not down the transom face.

If the boat has a speed above say 30 MPH and uses a good cupped prop, then you should be able to run higher ie you need to cut to more than 20 inches high. Please therefore specify motor size, prop type, boat size and deadrise and expected boat speed and we can give a more informed guess.

Hope this helps.

nevd

[Baldur]

I dug around in my drawings and found the old plans for this boat and it says the transom is 25 inches high vertical. Does this sound too tall? I have found (by trial, error and headache) that if a transom is too tall, it will cause the prop to cavitate when cornering sharply.

20 ft
mild-V
dory style
wood, ply on frame
40 HP
speed is not important, just keeping the prop working at its ideal depth is.

Thanks

[Baldur]

What happens if the prop is too deep?

[marshmat]

25" is also a common shaft length. Sometimes called X-Long or XL Shaft, this is usually seen on motors over 40hp, or on sailboat motors.

Shaft lengths are nominally 15" (short), 20" (long), 25" (x-long). In reality short can be up to 17", long is anywhere from 19" to 22". Measure your motor. (Most err on the too-long side.)

For most boats you want the antiventilation plate (the big one above the prop) to be level with the hull bottom on centre. On faster boats you can often run a bit higher.

Running too high causes ventilation, not cavitation. Ventilation refers to air getting sucked into the prop arc; cavitation is a more complex condition involving premature disruption of laminar flow over the blades (usually caused by a prop spinning too fast for its forward speed).

Running too low causes excess drag, since the housing above the plate is not as streamlined (and has no business being down there anyway). In some cases this can lug the engine, like running a car in too high a gear, or overload it.

In some boats it can be very dangerous to cut a transom lower than its design height. If your boat does not have a full motor well, it is better to put the motor on an external bracket or add a motor well if you need to cut down the transom.

[Baldur]

Thanks a bunch matt,
Ventilation not Cavitation. One day I might get all this ship lingo, but at least you knew what I was trying to say.

I am going to rebuild the transom to the 25 inch height as stated on the "outboard options" for some similar plans. Then I will find a motor to suit.
What I am trying to do is convert a well type to outboard type motor.

I am a kinda "reckless builder", not a lot of in-depth knowledge but i got plenty of tools. lol