Squirt with a long shaft 9.9

[CraigBBP]

I am close to finishing my Squirt. I have had a hard time finding a short shaft motor with remote. I purchased a 9.9 long shaft and the issue is my transom is 15"... Failed to think ahead at the start of the project! I have read many posts regarding a long shaft with a short transom and there does not seem to be a definitive answer as to if it will perform ok or not? I have thought about a jack plate although I am not sure it will look very good and what I may need to do to the transom to compensate for any additional torque... Any thoughts / experience would be greatly appreciated.

Thanks,

Craig

[hoodman]

I'm sure you could add to the height of the transom in the area where the motor bolts on. Maybe a block on top of the transom and laminated on the inside and outside with additional layers of 1/4" or 3/8" plywood?

[galamb]

The only "danger" in running a long shaft on a short transom is the extra 5" of lower unit down in the water.

As long as you are careful in shallow water and remember to tilt "early" if you are beaching it there is no "measurable" disadvantage to running the long shaft and there is no mechanical impairment etc.

Yes, it will produce a little more drag but seriously, we are talking about a 9.9 here on a light hull. A dirty bottom would probably create more resistance to forward motion than 5" of exhaust housing.

PS - should have added. Yes, there is a little more torque on the transom because the longer shaft gives a little more leverage to the thrust that is applied to the transom - but again, not enough that I would be concerned. (I repaired portable outboards/25hp and under and rigged small boats for many years).

There is no direct conversion from horsepower to thrust (pounds) but 1 hp is roughly 27 lbs of thrust. So the 9.9 if you pegged the throttle at the hole shot would be making about 270'ish pounds of thrust which is transferred to the transom to push the boat forward. In effect the prop is trying to crawl it's way "under" the boat and it torquing the motor bracket, attached to the transom "backwards" - since it won't move it ends up as forward thrust. The extra 5" of leverage can probably be calculated as a stress value but that math is somewhat beyond me If you are really concerned I would strengthen the "knee", not the transom itself which is probably significantly overbuilt just as a matter of design.

[Hercdrvr]

A 9.9 hp won’t hurt a Squirt. Do you have a second 9.9? Twins would look great on a Squirt.
Matt B

[CraigBBP]

The only "danger" in running a long shaft on a short transom is the extra 5" of lower unit down in the water.

As long as you are careful in shallow water and remember to tilt "early" if you are beaching it there is no "measurable" disadvantage to running the long shaft and there is no mechanical impairment etc.

Yes, it will produce a little more drag but seriously, we are talking about a 9.9 here on a light hull. A dirty bottom would probably create more resistance to forward motion than 5" of exhaust housing.

PS - should have added. Yes, there is a little more torque on the transom because the longer shaft gives a little more leverage to the thrust that is applied to the transom - but again, not enough that I would be concerned. (I repaired portable outboards/25hp and under and rigged small boats for many years).

There is no direct conversion from horsepower to thrust (pounds) but 1 hp is roughly 27 lbs of thrust. So the 9.9 if you pegged the throttle at the hole shot would be making about 270'ish pounds of thrust which is transferred to the transom to push the boat forward. In effect the prop is trying to crawl it's way "under" the boat and it torquing the motor bracket, attached to the transom "backwards" - since it won't move it ends up as forward thrust. The extra 5" of leverage can probably be calculated as a stress value but that math is somewhat beyond me If you are really concerned I would strengthen the "knee", not the transom itself which is probably significantly overbuilt just as a matter of design.

Torque on Transom.jpg

Thank you Graham,

I think I will add a little more to the transom knee although I do believe the whole rig is plenty strong as is.

I will test the boat with the motor on the 15" transom and see how it performs... Should it have issues I will add to transom or try a jack plate.

Craig

[BillW]

If the Squirt is built per the plans, there is no need to modify the transom knee.
That would be major surgery and/or result in looking funny.

Bear in mind, the transom is braced by the cockpit coamings, as well as the knee. In addition to all the other parts.
That boat is very strong. Some have been known to carry much more weight, and power.

Bill
(a Squirt builder)

[CraigBBP]

If the Squirt is built per the plans, there is no need to modify the transom knee.
That would be major surgery and/or result in looking funny.

Bear in mind, the transom is braced by the cockpit coamings, as well as the knee. In addition to all the other parts.
That boat is very strong. Some have been known to carry much more weight, and power.

Bill
(a Squirt builder)

Thanks Bill,

I just finished installing all the topside parts... coaming etc... I agree it is quite solid. I will leave it as is and try the long shaft on the 15" transom... Could not pass up the deal with remote etc included.

Thanks,

Craig

[CraigBBP]

The only "danger" in running a long shaft on a short transom is the extra 5" of lower unit down in the water.

As long as you are careful in shallow water and remember to tilt "early" if you are beaching it there is no "measurable" disadvantage to running the long shaft and there is no mechanical impairment etc.

Yes, it will produce a little more drag but seriously, we are talking about a 9.9 here on a light hull. A dirty bottom would probably create more resistance to forward motion than 5" of exhaust housing.

PS - should have added. Yes, there is a little more torque on the transom because the longer shaft gives a little more leverage to the thrust that is applied to the transom - but again, not enough that I would be concerned. (I repaired portable outboards/25hp and under and rigged small boats for many years).

There is no direct conversion from horsepower to thrust (pounds) but 1 hp is roughly 27 lbs of thrust. So the 9.9 if you pegged the throttle at the hole shot would be making about 270'ish pounds of thrust which is transferred to the transom to push the boat forward. In effect the prop is trying to crawl it's way "under" the boat and it torquing the motor bracket, attached to the transom "backwards" - since it won't move it ends up as forward thrust. The extra 5" of leverage can probably be calculated as a stress value but that math is somewhat beyond me If you are really concerned I would strengthen the "knee", not the transom itself which is probably significantly overbuilt just as a matter of design.

Torque on Transom.jpg

Hey Graham,

My marina is not trying to sell me a jack plate. They just don’t believe it will operate well? What do you think? I really don’t want to add to the transom or use a jack plate as the boat looks so nice in its intended state.

Are you pretty confident it will operate well?

I trust your experience and knowledge.

Thanks in advance.

[galamb]

I'm certain there is no mechanical reason that it won't work well. In fact, if you didn't know that a long shaft was installed I would bet that no one at that dealer would be able to tell if they were on the water underway.

I advise on another website (marineengine.com) and we have flogged to death the debate over long shaft on short transom. The one point that is often noted is that the steering feels a little "different" (un-scientific and un-qualified responses) with a long shaft on a short transom. I (suspect) this is due to the (extra) leg acting a little like a keel because when on plane there is just more in the water than would be with a short shaft - so could leave the steering a little more responsive and less forgiving if you cranked the wheel over hard (same type of reaction with motors running hydrofoil stablizers/whale tales on the cavitation plate (I have run one of those to keep the bow down at low speed in Trent/Severn Canal system - it does really change the steering characteristics but you adapt quickly because it becomes "normal"). And it will run a little slower and burn a little more gas because of the additional drag. Only you can decide if you can live with that.

So I would run it and if you have further concerns then try a simple jack/transom elevator (google T&R Marine Transom Elevator) and see if you notice a difference. But if you can live with drafting an extra 5" I wouldn't worry about it.

(edit) - I should have added that I am only talking about small outboards such as this 6/8/9.9's and not a 40, 50, or 125. I have no experience or have had no discussions as to any handling issues with a larger outboard with more shaft than required.