Tender Outboard Size. 15HP or 20HP?

[YPSILANTI]

I have a semi rigid inflatable for my 50 ft Catamaran. It was previously powered by a 15 HP two stroke Yamaha which I need to replace. I found the 15HP a bit underpowered. It would plane with 2 adults but not 4. I would prefer a 25 HP to a 20 HP but am concerned about the extra weight when removing the motor, beaching etc. The ability to waterski behind a 25HP is a large advantage. The difference in weight is apparently 7kg with the 25 weighing 57kg (125.6 pounds) . I have recently installed a dedicated swing arm and hoist with an upper and lower double pulley system for removing the motor. Have any of you had any experience when comparing the maneuverability and weight handling of a 20 HP vs 25 HP . Thank you.

[arsenelupiga]

consider leaving motor on dinghy during passages. See IMPI youtube channel Lagoon 44, 6 years cruising, no issues with decent size outboard.

Actually, one can at least on Lagoon tighten dingy for passages to have engine on and also, heavier dinghy as prescribed due to extra lines that keep it tight. No lift needed and saved weight can go towards extra dinghy size/power

I calculated for mine that can have extra 40 kg because of setup that helps with taking extra weight.

[Dave_S]

I have a 15hp Yamaha 2 stroke on my rib. It played up and I put a Honda 20hp 4 stroke on from another boat. It is slower to get on the plane. Stay with the 15 or go bigger than 20. I'll put the 15 back on ASAP.

[YPSILANTI]

Interesting replies. Much appreciated. I note that Impi has a 30 HP. Ive emailed Brent for his comment. Id be loath to not remove the outboard in serious weather or an ocean crossing. His 30HP certainly makes a 25HP more attractive for me than a 20 hp. Thanks so much.

[arsenelupiga]

Originally Posted by YPSILANTI

Interesting replies. Much appreciated. I note that Impi has a 30 HP. Ive emailed Brent for his comment. Id be loath to not remove the outboard in serious weather or an ocean crossing. His 30HP certainly makes a 25HP more attractive for me than a 20 hp. Thanks so much.

I followed Brent's lead. Nothing is 100% but looks pretty solid setup to me. Zero movement in seaway.

Test run here

[YPSILANTI]

Well, you've certainly convinced me to do likewise. I have the added advantage that my tender has a base onto which it sits and is hence supported by the base as well as the David's. That makes it very secure. Thanks so much for the pics.



Regards
Mike

[rom]

Originally Posted by Dave_S

I have a 15hp Yamaha 2 stroke on my rib. It played up and I put a Honda 20hp 4 stroke on from another boat. It is slower to get on the plane. Stay with the 15 or go bigger than 20. I'll put the 15 back on ASAP.

The 2 stroke has its advantages but what about noise, pollution, fuel consumption and having to to do the oil & gasoline mix ? of course there is also weight, reliability, ease of maintenance, etc ... Well, all in all 4 stroke seems to work better for me. I have a honda 20hp from 2011 that I use quite a lot, not much to complain about. I can do wakeboard with 2 adults in the dinghy.

[Dave_S]

Originally Posted by rom

The 2 stroke has its advantages but what about noise, pollution, fuel consumption and having to to do the oil & gasoline mix ? of course there is also weight, reliability, ease of maintenance, etc ... Well, all in all 4 stroke seems to work better for me. I have a honda 20hp from 2011 that I use quite a lot, not much to complain about. I can do wakeboard with 2 adults in the dinghy.

Agreed, except the 2 stroke is 10kgs lighter and there is no chance either of mine would pull a skier out of the water

[Navicula]

Originally Posted by Dave_S

I have a 15hp Yamaha 2 stroke on my rib. It played up and I put a Honda 20hp 4 stroke on from another boat. It is slower to get on the plane. Stay with the 15 or go bigger than 20. I'll put the 15 back on ASAP.

but how does the 20 and the 15 compare when you have 4 adults in it with grocerys and a jug of water??

I also need to make a decision like the original poster

[a64pilot]

Depends on the engine, the Suzuki 15 and 20 are the same engine, just different computers. Weight is identical at 99 lbs. for me the question was mute, why would anyone not spend just a little more for the 20?
When compared to the 15 HP two stroke Yamaha Enduro, the Suzuki 20 per HP is even lighter than the two stroke. Not total weight, but lb per HP.
The little Zuk is also in my opinion a better motor than either the Honda or the Yamaha. The fuel injection seems immune to ethanol fuels, and makes the engine run as well as it can, much better than a carburetor, just like what fuel injection did to cars. Remember the old carburetor cars, hard starting in the cold and didn’t run well until warmed up?

Just as a general rule, two strokes have more torque and a better “hole shot” than a four stroke. However just taking a motor from one boat to another may not have the motor properly propped. My Zuk came with an aluminum 10” pitch. It jumped onto plane but hit the rev limiter at about 25 kts or so. I bought a 12” SS Solas prop. It would run 30+ kts with just me aboard, but with the Wife and I crawled onto plane, had us leaning forward to get onto plane, so I got an 11” SS Solas. It now planes with both of us and groceries, and still will run 30 kts. I’m convinced that a 10” SS Solas prop would be perfect, that there is a lot more cup on the Solas blades so that it’s 10” is more pitch than a factory 10” aluminum prop, but I don’t think I’m going to buy another prop.
Point is having a motor correctly propped can make a huge difference, and it’s unlikely the factory prop just happens to be the perfect prop for your boat, odds are just against that.

Turn the clock back 40+ yrs to my Fathers old ski boat, we had two props, a three blade that would pull a half dozen skiers, and a two blade that was fast, really fast, but would only pull maybe two skiers and if the were big guys you had to get up on two skis and drop one, but the three blade would pull several up on slalom at once, the difference of course was speed. We had the first 100 HP Merc, 6 cyl engine on a little 16” Boat I think it was.
Props make a huge difference.

[Mango51]

i have a Yamaha 25 two stroke. 111 lb I hate the weight but love the torque and speed.

[YPSILANTI]

Aha Mango1, you have exactly what I'm contemplating. Do you find the weight a problem? Would a 20HP that is 16 pounds lighter be a better option? Would you go for the same motor again if you replaced it tomorrow or would you go lighter? Do you remove the motor and secure it to your yacht when underway and do you have a dedicated arm and pulleys for doing so? Any other info will be appreciated.

Regards
Mike

[MartinF]

You chaps did maybe 100 miles at 5kn to get to the anchorage. Why when you want to go a mile to the quay is it suddenly necessary to do 25kn??

[Dave22q]

My minority opinion is that for travel under a few NM both sizes are way too big. Smaller engines are lighter hence easier to remove and store. The time saved by going on plane for short distances is trivial but the associated loss of stability is not. Even in strong tides you do not need a lot of power because RIB's have low displacement. 5 or 10 is plenty for an average tender.

[Mango51]

Originally Posted by YPSILANTI

Aha Mango1, you have exactly what I'm contemplating. Do you find the weight a problem? Would a 20HP that is 16 pounds lighter be a better option? Would you go for the same motor again if you replaced it tomorrow or would you go lighter? Do you remove the motor and secure it to your yacht when underway and do you have a dedicated arm and pulleys for doing so? Any other info will be appreciated.

Regards
Mike

I have no davits. So I lower it with my mizzen boom. Is it to heavy? Yes. I don't think 20 lb makes a differens. I never go full speed but with 5 adults full of equipment it's nice to have extra power. For days I don't want to deal with the weight or long distances I have a torqeedo 3 hp. If i had to pick a replacement I would get the new 4 stroke 25 Yamaha.