Small 4-stroke motor on a 2-stroke bracket?

[mcaswell]

I'm helping a friend get set up with an outboard for his 21' daysailer. We need a bracket, but the transom is angled. I found a particular Garelick bracket that features an adjustment for the transom angle (so that the mounting pad can be close to vertical), but the description says it's specifically for a 2-stroke motor.

The motor he'll be getting is a 4-stroke 5hp that weights around 55lbs. Is it really not possible/advisable to put a 5hp 55lb. 4-stroke outboard on a bracket that is for a 2-stroke motor up to 115lb.s and 20hp?

--Michael

 

[Ross]

My bet is that the maximum would be 20 hp in a two stroke engine. 20 hp in a four stroke engine would exceed the maximum weight rating. The thrust from either should be about the same.

 

[sailingdog]

A four-stroke 5 HP motor should be just fine on a bracket rated for a two-stroke 20 HP motor, since the motor is smaller, less powerful and lighter than the maximum the bracket is designed for.

The big problem with using the bracket is when you're at the higher HP ranges, since four-stroke engines are considerably heavier than the two-stroke models.

 

[HukilauMike]

My understanding is that the two-stroke v. four stroke rating for brackets is not only related to engine weight; its related to the torque produced by the engine. Four stroke engines generate more torque (thrust) than their two stroke cousins. This is why you can use a lower horsepower four stroke model than a two stroke model for any given application. I don't pretend to be an expert in this subject, so I can't really offer an opinion if the combination of the weight and torque of a small four stroke is equal to the weight and torque of a larger two stroke. My suggestion is to contact the bracket's manufacturer and ask your question.

If the brackets rated for four stroke engines can't be adjusted to account for the angle of the transom on your friend's boat, another option you can explore is to put an angled mount pad in between the bracket and the transom. You can easily make one of these from Starboard or scrap wood coated with epoxy. Several posters here have done so; there are pictures of their work in the archives.

 

[Ross]

Mike, Engines come equiped with gear cases so that the combination of crankshaft speed and torque can be taylored to the needs of the application. HP is a product of torque and rotational speed.

 

[Benny]

Never new there were dedicated motor mounts for 2 and 4 stroke engines. I just went by weight capacity and my personal test. I just tugged hard at it with my hands and body weight and if it did not come off or break it was good enough. I most have been just lucky all these years.

 

[tommays]

All outboards are rated in prop shaft HP

 

[mcaswell]

Never new there were dedicated motor mounts for 2 and 4 stroke engines. I just went by weight capacity and my personal test. I just tugged hard at it with my hands and body weight and if it did not come off or break it was good enough. I most have been just lucky all these years.

My guess, as the other replies have alluded to, was that perhaps someone bought a bracket designed for a 30hp 2-stroke, put a heavier 30hp 4-stroke motor on it, and the bracket broke.

Seems like they could simply adjust their descriptions of the brackets... emphasize the weight limit, and maybe give separate horsepower ranges for each type of motor ("up to 30hp 2-stroke, or 20hp 4-stroke", for instance).

--Michael

 

[Manny]

I spoke to one of the outboard bracket manufacturers a few years back and they strongly recommended that a four stroke not be put on a two stroke bracket due to the torque even if it is under the weight an hp limits...
Check the outboard, they usually have an angle adjustment. You may not need a bracket with an adjustable angle.

Manny

 

[druid]

Hmmm.... lots of misinformation floating around. A reply to my post about 8hp 2-stroke vs 6hp 4-stroke said a 2-stoke has more torque. At max RPM, they're about the same for a given hp rating, since they both achieve that at the same RPM. Torque at low RPM is not significant for the bracket, since the most stress is at max RPM.

The OP said he's mounting a 4-stroke 5hp on a bracket rated for 20hp. I seriously doubt even a 4-stroke 5hp puts out more torque than a 20hp.

I've also heard "don't put a 4-stroke on our brackets!" but the only explanation as to why was weight. And again: a 4-stroke 5hp will weigh much less than a 2-stroke 20hp.

I think Micheal's right: somebody put a 20hp 4-stroke on a "20hp" bracket and it broke and he sued, so now the mfrs are all saying "no 4-strokes!".

As long as you're under the hp and weight limits, you should be fine.

druid

 

[OdayDave]

I bought a Garelick motor mount to replace the original mount on my O'Day 222 several years ago when I was still using a 2-stroke 7.5 hp Mercury. The Garelick allowed me to remove the wedge that went with the old motor mount and put the new one directly on the transom. Then the old motor died on my so I replaced it with a 6 hp 4-stroke Tohatsu. The Tohatsu actually is lighter than the Mercury (I save about 20 lbs). I know that the particular Garelick mount is not rated for 4-stroke engines, and I know that Garelick would advise against it. However, it would be costly overkill in my mind to replace it on that basis. I do look at the mount from time to time to make sure it is not showing any signs of stress. So are you taking a risk by going with the mount that is not rated for a 4-stroke. Sure. But I suspect that these are overengineered so that the risk is very low. I know I don't lose any sleep over the choice I made.

I just added a couple of pictures. Note that I added a small spacer so that the handle didn't bump the rub-rail when in the "up position".

Dave
O'Day 222