Depth of motor mount

[crosscuts]

Some time ago there was a discussion about the height of a transom to mount an outboard motor. I can't find the thread. I am rebuilding a 16' wood/canvas square stern. The top of the transom is broken off. Could someone please tell me an ideal measurement from the top edge of the transom to the bottom planking?

I hope to mount an old 2-3 hp motor.

Many thanks for any help.

R.C.

 

[Dan Lindberg]

Well, the standard length motor shafts are, short (15"), long (20") and "extra" long (25"?) (not sure the correct name for these.)

What length is the motor you plan to use?

Dan

 

[crosscuts]

Transom Height

Thanks for the response Dan, I don't have a motor yet but I will look for an old one to match the boat. I am guessing the square stern is late 20's or early 30's. I dummied up a reconstructed transom and 15" looks about right.

R.C.

 

[shelldrake]

R.C.

I'm working on a Candian Canoe Co 16' vee stern. I can measure the transom height for you, but I'm not sure what measurement you're looking for. With regard to motor shaft length, the critical measurement is transom top to waterline and I'm not sure where the waterline is on this boat (and of course, that will vary according to the load).

Matt

 

[shelldrake]

R.C.

I just went out and measured from the keel bottom to the top of the transom......approx 17". I say approx because the keel slopes upward to the transom from the flat run along the bottom on this boat.

Hope this helps.

Matt

 

[crosscuts]

Square stern

Thanks for all the advice and information. It all gives me a basis for a good guess. I don't know how the boat will ride in the water and have no idea what the specs on whatever old motor I find will be. I will rebuild the transom so it is pleasing to the eye and hope the prop is in the water.

R.C.

 

[Dan Lindberg]

RC,

I 2ed Peter's post.

And I'd hazzard a guess that if you pick motor from 50's or later, it will likely be a 15" length. I don't know about older motors. Long shafts on small motors really didn't get popular until maybe the 70's(?), at least on MN waters.

And a thought, if there happens to be an old outboard "dealer" or old boat club in your area, you might be able to look at a few old motors of the size/brand you might use and measure the distance from the cavatation plate to the top of the transom.

Dan

 

[Alm]

... and "extra" long (25"?) (not sure the correct name for these.)

Must be Longtail Motors or Mud Motors http://www.northfloridamudmotor.com/home - for very specific boating needs, and they are expensive.

This is probably that 2nd of 2 threads going at the same that you mentioned.

Old 2-3 hp motor will be likely 2-stroke, not meeting today's exhaust standards in North America and therefore not sold by dealers here anymore, - but he can probably operate it if finds one in a good shape. I don't think that all 2-stoke owners have thrown their motors away when stricter standards were introduced...

 

[goldencub]

A couple more thoughts on the subject - the transom height measurement is from the top to the bottom of the boat (or keel, if there is one) and not to the waterline. The reason is that at speed, the water level will, in effect, be even with the boat bottom, at least in the short space between the back of the boat and the motor lower unit. That said, your boat and contemplated motor combination might not move forward fast enough to cause this situation. The important thing is that the lower unit (below the cavitation plate) be below the surrounding water during all phases of operation. I would say that anything around 15 inches that pleases your eye will be fine with a "short shaft" motor of 2 -3 hp.

And finally, there are thousands of old motors in use, carefully restored and maintained by members of the Antique Outboard Motor Club, Inc. (AOMCI) - a very active organization not at all unlike the WHCA in their activity and entbhusiasm with local chapters all over the country. There will be some in your area, and they would be very helpful in your search for an appropriate motor. Look them up on the web to find a chapter near you. I matched a 1949 Martin outboard to my 1949 Sebago wood and canvas cartopper. Good luck. Al

 

[Alm]

.... The important thing is that the lower unit (below the cavitation plate) be below the surrounding water during all phases of operation.

I've been told that due to small size of cavitation plate small 2-3 hp motors are the worst in sense of "ventilating", i.e. when the motor at high speed creates a "hole" in the water surrounding the prop, and prop is catching the air, reducing efficiency. Especially on canoes without wide transom. I wonder if this cav-plate extender would help - anybody using it? http://www.savvyboater.com/p-76-hydro-tail-performance-stabilizer.aspx (don't understand how it can be 10" from top to bottom as they say). Somebody made a similar fairing for his outrigger canoe, because his motor was side-mounted - but he made it above the cav-plate, and this one in the link looks like going from the cav-plate down.

 

[goldencub]

Alm - I can't speak with engineering authority, that's for sure, but I think you're worrying too much about cavitation. You're going to be operating at very "genteel" speeds with your proposed boat and motor sizes. Water skiing and hydroplaning or "getting up on the step" are not in your equation and those cavitation plates are meant for those situations. Yes, you might experience a cavitation episode, but it would be if you abruptly changed course in your upper speed ranges - and I mean abruptly!!! You should have no problem within the normal envelope of operation due to your low speeds and horsepower. If it should happen, just back off the throttle for a second or two to let the prop bite some clean water and then put the power back to her! It will probably never be a dangerous situation. If I am way off base here, I hope someone will jump in and correct me!!! Al

 

[Todd Bradshaw]

My guess is that Goldencub is on the right track. We are talking about a canoe here. They don't need a lot of power to move pretty nicely, and they're generally not thought of as high-performance powerboats.

We had a 12' Avon inflatable with an 18 hp Nissan on the transom and I tried a Doel-Fin (the original brand of those bolt-on fins) on it for a while. Their main goal, by far, is to get the boat up on a plane faster (lifting-wing, airplane-style applied to the back end of the boat). They also tend to flatten the boat out when making turns in situations where it would normally be leaning to one side. That part felt pretty strange, doing high-speed turns level. I suppose it's perfectly safe, but it felt sort of un-natural.

Unless you're a pretty wild and crazy guy, your motorized canoe will likely never get up to a speed where that thing is going to be doing much of anything other than adding to drag. It's also kind of a pain in terms of storage, handling, etc. to have something bolted to your outboard that makes the lower unit 20" wide.

 

[Rollin Thurlow]

the very general rule of thumb I use is; a short shaft for a transom dept of 17" or less. Anything else needs a long shaft.