Outboard mounting height

[Heron]

Good Day All, being somewhat of a boating neophyte I have a perplexing (to me) question for this "Group in the Know".

I picked up a Smoker Craft 15' Alaskan and have re-powered it with a new long shaft Honda 20 hp. The boats transom measures 20" but with the motor mounted the cavitation plate is 2 7/8" below the boat bottom. There is a center rib that runs the length of the boat to within 4" of the transom that adds 1 1/4" to the equation. Which brings me to two questions:

1. Should I raise the outboard 2 7/8" to the actual boat bottom or 1 5/8" to account for the rib.
2. The bigger question is; will doing this make any discernible difference in top speed or performance?

I'm sure there is a reason for the extra length in the shaft but I don't completely understand why the cavitation plate on a new outboard wouldn't match up perfectly.

Thank everyone in advance for your time and input.

 

[ktoelke54]

My guess is that manufacturers of outboards give you a little extra length because there are fewer problems running an outboard a little low than high. If to high you’ll not only suffer prop blowout but run the risk of not having intake of your cooling system fully submerged, thus overheating. I’ve done a lot of fiddling with motors in the 15-25 Hp range getting the height just right. I’ve found that the center rib does count. I had one motor that would blowout with the cavitation plate level with the hull, I didn’t count the height of the rib. I lowered it an inch, end of problem. I ran another motor on the same hull, cavitation plate level, no blowout. So it depends on the particular motor, prop, etc. If you have a way of experimenting, without drilling holes in your transom, that’s the way to go. If not, to be safe, I’d consider the rib the bottom and level your cavitation plate with that. Performance gains with raising to the proper height have been modest but noteworthy, maybe 1 mph.


Sent from my iPad using Tapatalk

 

[RaisedByWolves]

With the right prop and motor height you can get considerable gains.

Motor height alone as stated above is good for 1-2 mph, but the real benefit is in proper plaining and handling.

I keep raising the motor while watching for proper cooling and take it up to where it will start to blow out in turns or chop, then go back down a half inch.

Half inch increments are what I shoot for.

 

[JL8Jeff]

Is the motor mounted to the transom by clamping it on or are there holes in the transom and bolts going through the motor mounting plate? My jet outboard is a 15" short shaft motor and I have it mounted at least 2" above the transom but it's bolted through the transom. If yours is bolted through the transom, you can raise it 1 hole at a time and test it out, that's what I ended up doing. I kept going up until I got spray back over the transom (that's a jet outboard issue) and performance slowed down. Then I went back down a hole and it's been there for years.

 

[turbotodd]

that is normal for honda they are a little longer than 20"

put it on a mini-jacker

 

[Heron]

Thank You all very much for the information.

I raised the motor 1" to start with. I'll get her in the water later this week.

 

[Heron]

Well I finally got the boat out and gave it a run. Early in the day with flat water the 1" rise seemed to be the ticket. I had minimal gain in top speed, maybe 1 to 1.5 mph, and it seemed to come on plane a bit sooner. But later in the day with a chop on the water and moderate waves, at top speed (20 mph) the motor would start to cavitate a bit.

The motors cavitation plate is still 1/2" to 3/4" below the center rib so I was a bit surprised by this result. Think I'm going to drop it back to original and call it good.

Thanks again for all the input, greatly appreciated.

 

[nccatfisher]

Well I finally got the boat out and gave it a run. Early in the day with flat water the 1" rise seemed to be the ticket. I had minimal gain in top speed, maybe 1 to 1.5 mph, and it seemed to come on plane a bit sooner. But later in the day with a chop on the water and moderate waves, at top speed (20 mph) the motor would start to cavitate a bit.

The motors cavitation plate is still 1/2" to 3/4" below the center rib so I was a bit surprised by this result. Think I'm going to drop it back to original and call it good.

Thanks again for all the input, greatly appreciated.

How did you have the motor trimmed? With the plate that low I would work on the trim before I set it back down.

 

[Heron]

I have not changed the trim at all. The motor curenntly seems to be set at an 8-10 degree angle in (towards the transom). This seems a little much but I don't know for sure. I'll move the adjusting pin up a notch and give it a shot.

What angle should I shoot for in relation to the boat bottom?

 

[RaisedByWolves]

I have not changed the trim at all. The motor curenntly seems to be set at an 8-10 degree angle in (towards the transom). This seems a little much but I don't know for sure. I'll move the adjusting pin up a notch and give it a shot.

What angle should I shoot for in relation to the boat bottom?

Theres a whole science to this and its a lot of trial and error. The deeper your prop, the more negative angle you will need to counter proposing. The higher your prop the more positive angle you can add in to get the hull out of the water more. More positive angle also moves the prop rearward and into cleaner water which may help with the cavitation.


If "I" were in your shoes (And I am) "I" would call where you just had it good and go up an inch or so on the prop pitch.


Im on my 4th prop (14' with a 25hp evinrude/15P prop) and need to raise my motor another 1/2-1" to get it right.

Your boat, how its loaded and powered, and how you intend to use it are the variables to be considered.

Theres no "One size fits all" recommendation, its all down to how far you want to take it.

 

[CedarRiverScooter]

Would a whale tail help with this problem?