Dink OB - Long Shaft vs Short Shaft

[44'cruisingcat]

Maybe you need to go back and actually read the replies.


Nobody has criticized you. What has been said is that the best option would be the correct shaft length, but if it's not readily available, or affordable, ie if the long shaft motor is a bargain, it will still work, so don't just discount it out of hand.

[zeehag]

i use longshaft engines with my ribs, but now i row, so...i donot like short shaft engines due to cavitation issues.... and potential use on a sailboat--doesnt work, so mine are generally long shaft. i had a 15 hp short once--was an excellent engine until our neighborhood outboard killer put sea water in fuel........ that was fun.....

[Sailmonkey]

The only downside I've ever seen to a long shaft on a dinghy has been a power steering type of tendency when on a plane. Only dangerous if you're not ready for it, but like everything supposedly dangerous you get used to it and learn to overcome it.


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[a64pilot]

If you have a planing hull, the anti-cavitation plate should ride right on top of the water, you should see the top of it, dry when on plane.
Deeper than that and you lose significant performance, sort of like having the weight and fuel burn of a 15 HP, but the output of a 9.9. If your cool with that, then fine no problem.
But do make sure it's not a high thrust motor, they turn a larger prop at lower RPM to push a big boat, and while they will work too, their performance loss will be even greater.
I'd opt for the "proper" length motor, the one that both the boat and engine builder recommend. But then I'm also the type to "prop" even a dink, that is spend money to ensure the motor will reach full RPM but no more

[Sailor Doug]

Any body ever notice a dink with long shaft heels suddenly if motor gunned at idle? May lose a guest or child.



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[Terra Nova]

Originally Posted by Sailor Doug

Any body ever notice a dink with long shaft heels suddenly if motor gunned at idle? May lose a guest or child...

This is potentially the most significant problem: applying power with the motor turned~ 90*, especially from a standstill; depends on the dinghy. Everyone who compromised because of price also compromised their guests' safety.

Long-shaft outboards, in dinghy sizes, are usually more rare.

[44'cruisingcat]

Originally Posted by Sailor Doug

Any body ever notice a dink with long shaft heels suddenly if motor gunned at idle? May lose a guest or child.



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Nope. Used this motor on 3 dinghies now, done about 3000 hours, and no, I really can't say I've noticed that. Haven't lost any guests or children either.

[44'cruisingcat]

Originally Posted by Terra Nova

This is potentially the most significant problem: applying power with the motor turned~ 90*, especially from a standstill; depends on the dinghy. Everyone who compromised because of price also compromised their guests' safety.

Long-shaft outboards, in dinghy sizes, are usually more rare.

Generally motors in the 15 hp range (as per the OP) can't turn anywhere near 90'.

But I'd suggest if you were able to apply power with the motor at 90' the boat would heel whether the motor was a long shaft or not.


And if you do regularly apply full throttle with the engine turned at 90', you're compromising safety for stupidity...

The snide comment about compromising on safety is pretty stupid. Do you drive the latest top of the range armour plated Mercedes? If not aren't you compromising passenger safety? Why do you not have a 100' yacht? 200' ? Another price/safety compromise? Do you carry a 2 man liferaft for when there are 2 aboard, a 3 man for when there are 3, a 4 man for when there are 4, etc etc... (A too big liferaft can be as dangerous as a too small one)


Compromising safety for price again?

[Sailor Doug]

Right long shaft, wrong dink, it can happen. Some people are lucky some aren't.


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[colemj]

Originally Posted by a64pilot

If you have a planing hull, the anti-cavitation plate should ride right on top of the water, you should see the top of it, dry when on plane.

Actually, the cavitation plate should be 0-3" below the bottom of the transom (as per every dinghy instruction manual I have ever seen). With deeper hulled boats, this isn't right at the surface when on plane. For example, neither of our ~12' RIBS had the cavitation plate that close to the surface due to the OB being properly placed on the transom according to the manufacturer's instructions.

(no longer specifically addressing a64 now):
I have driven tons of boats that had a "too long" shaft and never once experienced any "dangerous heeling" or other noticeable issue with them. In fact, people would take ours out unaware and never notice the difference.

I think some are not putting this in perspective and are relying on pedantic belief. Think about it - at MOST there is a 5" difference in prop depth from bottom of transom. Depending on the specific dinghy design, it may only be 2-3" difference, since short shafts are actually a bit too short for many dinghies (something that is actually worse for performance/safety, but nobody frets about that for some reason).

If one thinks that achieving an exact precise match is significant, then one has not much experience in the matter. Based on some of these comments, I suspect 44CC and I have years more experience with a far greater range of engine/boat matchups than others.

Mark

[Sailor Doug]

Originally Posted by colemj

Actually, the cavitation plate should be 0-3" below the bottom of the transom (as per every dinghy instruction manual I have ever seen). With deeper hulled boats, this isn't right at the surface when on plane. For example, neither of our ~12' RIBS had the cavitation plate that close to the surface due to the OB being properly placed on the transom according to the manufacturer's instructions.



(no longer specifically addressing a64 now):

I have driven tons of boats that had a "too long" shaft and never once experienced any "dangerous heeling" or other noticeable issue with them. In fact, people would take ours out unaware and never notice the difference.



I think some are not putting this in perspective and are relying on pedantic belief. Think about it - at MOST there is a 5" difference in prop depth from bottom of transom. Depending on the specific dinghy design, it may only be 2-3" difference, since short shafts are actually a bit too short for many dinghies (something that is actually worse for performance/safety, but nobody frets about that for some reason).



If one thinks that achieving an exact precise match is significant, then one has not much experience in the matter. Based on some of these comments, I suspect 44CC and I have years more experience with a far greater range of engine/boat matchups than others.



Mark

WOW how old are you? Bought my first zodiac in 1985 and used the wrong motor and learned a lesson, it was not suitable. Depending on motor angle the rotational torque (tipping effect) can be 50% greater on a long shaft motor if mounted on a transom designed for a short shaft motor. (Force x Distance)


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[a64pilot]

http://www.propgods.com/forum/Default.aspx?g=posts&t=98

On edit, I never said a prop being too deep won't work, the boat will move and the engine won't be damaged, but you will lose speed and efficiency due to having more motor in the water and therefore more drag, off plane difference is so small it won't make much difference, but if you plan on running say 20 Kts, it will matter. Waves permitting, 20 kts is a good comfortable speed in my dink.


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[Terra Nova]

Originally Posted by colemj

... I suspect 44CC and I have years more experience with a far greater range of engine/boat matchups than others.

Mark

Delusions of grandeur.

[MBWhite]

Wow, guess I need to remember to put long shaft/short shaft discussions up in the cat-monohull, best anchor. best brand of motor oil, etc category!

Personally, I've decided the best approach to take is, ahhh, errr... never mind.

[colemj]

Originally Posted by a64pilot

Engine Height before/after pics - prop questions - Propgods Forum

On edit, I never said a prop being too deep won't work, the boat will move and the engine won't be damaged, but you will lose speed and efficiency due to having more motor in the water and therefore more drag, off plane difference is so small it won't make much difference, but if you plan on running say 20 Kts, it will matter. Waves permitting, 20 kts is a good comfortable speed in my dink.

That link refers to a 50mph boat running two 200hp engines. In this application, the engine height and attitude is always matched perfectly to the boat. In fact, the engine mounts themselves have adjustments to them for this purpose.

For the typical dinghy application, this is not important. If it was, manufacturers would be providing adjustable mounts because there is a very wide range of possible dinghy model transom heights and outboard mount combinations.

It is also interesting to note in that link that raising the engine mount so the cavitation plate moved from below the surface to at the surface gained them 1.5mph on their 50mph boat.

Mark