25 hp on 9 ft sib

[ramminjammin]

Will be cruising Bahamas for three months this winter and need to power a small sib that i can transport with a 30 ft sailboat from s carolina to bahamas. Right now i have an 8' 10" Seabright air floor inflatable. I have a 2. 1/2 hp for around marinas and anchoring out. But need more power to get around with three adults for exploring , fishing etc.

The sib will take a 10 hp and engine weight of 202 lbs. So why not put on a 25 yamaha two stroke that weighs 106 lbs ? If i dont crank her wide open and use half its power, beside 26 lbs heavier than a 9.9 it is the same ? This would allow me to upgrade to larger sib or rib later and use same motor. I have seen 10 ft ribs on youtube with 25 hp motors, seems to work....just have to use only 60 % of its horsepower.
Anyone done this or have knowledge as to why or whynot this is doable ?
Thanks

[Nos4r2]

10hp would be more than enough.

25hp is suicidal. The torque will turn it over very easily.
25hp on a 10' rib is pushing it. On a sib it'd be inviting a Darwin award.

[ramminjammin]

Dont quite understand the physics, the boat is rated for 202 pound motor the 25 is 96 pounds under that weight....so just sitting there it should be ok. I dont think 10 hp will plane that rib with three adults aboard. I am 100% sure that in a headwind and wot she will do cartwheels. I would think with a goveneror on the throttle to 50% it is just the same as having a 12 1/2 hp fourstroke on there.

Also there might be the slight chance of flirting with Darwin, going solo in ideal conditions, it might be irrestible to see what she will do ....with a few tweaks is 50 mph possible ?

[Nos4r2]

Yeah, I re-read and edited that bit after I posted it.

You'll have so much torque though that even only going to half throttle you're likely to turn it over from a standing start. It's not all about HP.

Realistically, either buy a bigger boat or a smaller motor.

[willk]

Originally Posted by ramminjammin

I would think with a goveneror on the throttle to 50% it is just the same as having a 12 1/2 hp fourstroke on there.

Think again. Would putting a Porche engine in a VW Bug and restricting the throttle to 25% leave you with a car that handled safely?

Anyway - I think you may need a Trolling motor for that SIB...

Hey - whatever happened to Busboy? He was into this turbocharged SIB stuff! I'll dig his thread up.

[willk]

There ya are: http://www.rib.net/forum/f16/bbs-go-fast-sib-37876.html

[Fenlander]

I know it sounds a bit killjoy but...

Overpowering above the ideal balanced outfit makes for a very unsatisfactory experience.

If you look more carefully at the specs for yout boat the 202lbs is for engine, fuel & battery etc... the rating for engine on the transom alone is 132lbs.... OK your 25hp will still be inside that but closer to the limit.

I don't think taking the throttle range down to 50% will stop that initial grunt of a 25 and also it will be bad for the outboard to constantly run at such reduced revs... well less good anyway.

Flat air floors do not like being driven hard... you can tie them in knots and all sorts of odd flexing will happen as you raise both outboard power and weight above the ideal.

In addition three adults might be OK in a 2.7m SIB used as a tender but for going any distance on the plane and/or fishing is far from ideal.

Personally I'd go for something like the Tohatsu/Nissan 9.8 2-stroke on your current SIB... or buy a bigger SIB.

[The Gurnard]

Hi ramminjammin..perhaps this is worth a thought ?

That brand of inflatable seemed to bring back a memory of a guy on another forum who had the 4.3m HD version. I presume the boat you refer to is this one ..

seabright marine, seabrightmarine, inflatable boats, inflatable boat, hypalon, RIB, RIBs, hypalon inflatable boat, hypalon inflatable boats, hypalon rib, hypalon ribs

Just for info.. The guy could not get his Seabright 4.3 HD to plane with 2 adults and a 20hp. After much discussion and correspondence.. the dealer who sold the Seabright..a dealer in Canada..admitted it needed a revised shaped keel..and sent on a “special” made far larger inflatable keel. It did plane after that.. but it shouldn’t take that kind of research or repeated correspondence to get a boat to work as advertised ?

Do some research yourself on that brand..and I think you will find it is a very budget brand of inflatable. At the time..the forum found very little info on it .. I recall the only U Tube video we could find of one like the guys in question..was this one

Its called “a Seabrighr fun inflatable”.. watch the video ..it looks a lot of fun..doesnt it ?

https://www.youtube.com/watch?v=K7zLi8fuCaw

When a reputable airfloor like the Honwave 2.7 recommends an 8hp max Outboard (46kg weight)(105lb)

http://www.gaelforcemarine.co.uk/Ima...ons/T27IE2.pdf

I ask myself ..what does that slightly smaller Seabright have that makes it suitable for an engine up to 202lb ? Except perhaps that they have not got a clue what they are selling ? ( This is my opinion only based on the other guys experience and comparison with known brands)


As for putting a 25HP on that particular one ..I would be very wary

[jeffstevens763@g]

The boat is way out of spec for a 25 hp have a look at a 25 hp rated boat you will notice weight of boat, beam, tube diameter & length which all dictate carrying capacity & ability to plane safely way above your boats spec. A 15 hp would probably do what you want but better to swap for the set up you really want.

[Poly]

I used to think a little bit of extra power would do no real harm. Then I saw someone crack their transom by opening up a 15HP on a transom rated for 10. They didn't even get on the plane, never mind full speed.

[cgoing]

Sounds like a bad idea to me. Never seen a small
Inflatable like that which could take a 200 lb load on the transom. The extra power will probably cause damage over time.
Also be aware that the coast guard looks at horsepower ratings so if you get stopped with a motor that big it does not matter if it is under the weight rating or not. Being over the horsepower rating would be a violation. Here in New England they are serious about violations like that.

[jeffstevens763@g]

Originally Posted by Poly

I used to think a little bit of extra power would do no real harm. Then I saw someone crack their transom by opening up a 15HP on a transom rated for 10. They didn't even get on the plane, never mind full speed.

Not suprised poly takakat use to be rated to 15 hp but now down graded to 10 hp problems with the transom.
That's why manufactures give a maximum hp & a recommended hp.
Interestingly there's an articlal in power boat & rib two guys wild camping out of Largs in a takakat & propane engine for info

[ramminjammin]

Definatly going to purchase a larger rib and engine combo as you all have so kindly suggested. I will keep my 9ft with 2 hp for general anchoring out and accessing shore. I will then have a second runabout to do fishing, snorkling, beaches and exploring once out in paradise. Have been running the woodstove up here for a few weeks............

[Limecc]

I notice some guys are running jets instead of propellors. How does this affect the rating of the transom? If it's not overweight presumably you go off the rated power of the jet? A 40hp powerhead would typically be converted to running a 25hp jet for example.

If you are planning the equivalent of some off-road work exploring shallow rivers and tributaries this seems the way to go. I saw a fantastic series of Russian YouTube vids where they were going up natural weirs and over rapids. Incredible the beating those boats and engines took!

[ramminjammin]

Ended up buying used ab 10 ft al rib with 18 hp nissan

[Fenlander]

That will still be very lively but far better than the previous outfit you mentioned. Let us know how it goes!

[Gwozhog]

I run a 15 hp on a 310 Infl a table that's only rated for 10. Its the perfect engine.