Long Shaft Motor on Dinghy ?

[troppo]

My sailing boat has a 18 hp 2 stroke long shaft outboard. I am thinking of buying an aluminium dinghy and temporarily using the 18 hp on it. The dinghy can take a 20 hp but I am wondering about using a long shaft on a dinghy which really should take a short shaft. Is that a problem, apart from the prop being down further where it could more easily hit something?

[GordMay]

Yes , it is possible to use a longer shaft length than what is called for, provided there is sufficient clearance when the engine is tilted in the up position. You will, however, get a little more drag and a little less top end performance. Be extra careful when operating in shallow water.

[Captain Bill]

Besides what Gord said you should also be aware that because the prop is at the bottom of a longer lever arm it will have much more tendency to lift the bow. There is a distinct danger that if you give it a lot of throttle suddenly the dinghy will go over backwards, especially if no one is sitting in the bow. This actually happened to a friend of mine who put a long shaft motor on a dinghy designed for a short shaft. He wasn't hurt, but he dunked his engine and spilled everything in the dinghy into the drink. It was not good for the engine.

[troppo]

Thank you Gord and Bill. I'll take extra care in shallow water and avoid quick acceleration. Hmm, the tidal river I will use it on is full of shallow mud banks (which is why I want the motor on the dinghy so I can explore before moving my keel boat through) and plenty of crocs live there. If I see a croc I will have to remember not to twist that throttle too fast!

[SkiprJohn]

Our YMCA had a long shaft Honda 10 mounted on an Avon. The fellow that they bought it from extended the transom height to accomodate the engine. I'd not recommend doing that because it put a lot more stress on the transom and caused quicker failure.
kind regards,

[atoll]

weld on hydrofoils to the bottom of the dinghy...................

[Jim Cate]

G'Day Troppo,

I don't think that you will have a control problem with a tinnie... it will be heavy enough and have enough buoyancy aft to keep it from flipping. If it worries you, do some progressive experimentation with throttle application to see if it gets light forward.

Meanwhile, with that much hp on your Tophat you might be able to tow a waterskier!

Cheers,

Jim

[troppo]

Originally Posted by Jim Cate

. . . .

Meanwhile, with that much hp on your Tophat you might be able to tow a waterskier!

Cheers,

Jim

That's interesting, you are not the first person to suggest that. I have to say, since I am a solo boatie, that watersking is way too difficult. If I am on the skies, then there is no one in the cockpit watching for my hand signals. So, I just have to put up with using a tractor tube at the end of a long rope ladder. At the end of the straight I whip up the ladder to the cockpit and turn her around.

Actually, the motor came with the boat and I think I could do with a smaller motor, maybe 10 hp. Doesn't take much twist of the throttle to have her up to 6 knts on calm water though there is no problems of doing that even against high wind and waves, even 7 knts though the fuel goes down quickly.

[jeremiason]

Originally Posted by troppo

My sailing boat has a 18 hp 2 stroke long shaft outboard. I am thinking of buying an aluminium dinghy and temporarily using the 18 hp on it. The dinghy can take a 20 hp but I am wondering about using a long shaft on a dinghy which really should take a short shaft. Is that a problem, apart from the prop being down further where it could more easily hit something?

In addition to the above commments....

If you intend to use the 18 hp outbord on your new dinghy becareful about the weight. I wold assume that a long shaft motor will add more weight to the outboard.

Yes, the dinghy is rated for 20 hp, but it also has a maxium motor weight as well (Displayed on the data plate)... If you exceed tht weight, the transom will be lower in the water and cause splash over when you throttle down, which could swamp the dinghy. It will also put more stress on the transom while motoring.

This was especially problematic when four-strokes first came out, which weigh about 20-25% more than 2-Strokes...

People would purchase RIBS made to accomodate a 15 hp motor, only to find out the raft had been designed for an outboard of no more than 90 lbs (2-Stroke), but the new 4-Stroke 15 hp outboards weighed in at bout 115 pounds, which caused some up happy people.

[bruce smith]

In addition to the possibility of the "lever arm" pushing the bow too far up , it can also push the bow too far down.
The tinnie will have harder chines than the deflateable, and it may have a tendency to trip on its' chine.
So , be careful when you are slowing down in a turn .
And feel for the boat pulling to stbd at medium , or any , speed.

[kevin mcintosh]

Originally Posted by troppo

My sailing boat has a 18 hp 2 stroke long shaft outboard. I am thinking of buying an aluminium dinghy and temporarily using the 18 hp on it. The dinghy can take a 20 hp but I am wondering about using a long shaft on a dinghy which really should take a short shaft. Is that a problem, apart from the prop being down further where it could more easily hit something?

Cant see were it would be a problem just means your thrust and steering would be lower in the water also their would be no cavitation that you get on short shafts only issue i can see is when your comming into shore you will have to tilt your engine earlier i had a long shaft some years ago on a tinny and had no problem can you answer me a question please i have a tophat 25 mk 2 tried joining tophat forum to no success wanting to find information about these boats and maybe talk to fellow tophat owners also looking to track boat history as my boat is now at bribie island qld but has a tasmanian yacht club sticker on the window and it came from adelaide sa and has had a life raft fitted at some time and wind steering aparartiss on rear of boat kevin is my name 0438983900 krmcintosh@bigpond.com if you can help

[Ex-Calif]

Welcome aboard Kevin!

Considering the questions is over 2 years old they may have it sorted by now.

OTOH - We had 20hp on a 10 foot tinnie as a kid. We didn't have the internet to tell us how to use it so we just had a freakin' blast at WOT...

PS - you might consider starting a new thread in the mono section about the Tophat...