Yamaha 100hp vs 115hp

[Atlantic_Man]

Hi. Recently bought a Humber 5.5m RIB with a Yamaha 100hp outboard. Really happy with it. But, as the max weight outboard recommended is 115hp for the 5.5m, is it worth upgrading to 115hp for the extra speed? And any idea what the extra speed would be compared to the 100hp? Cheers.

[jonp]

Is the 100hp the old model or the current version ? The difference between the old 115 and the new one made a difference in economy and performance, so if going from the old 100 to the new 115 you would notice a difference, perhaps 4-5kts. Why not go to the 130 which is the same weight as the 115 and lighter than the old 100hp.
You could look at getting the 100 re-mapped, which is normally a case of removing the ecu box, sending it off and paying the money.
You will obviously need to re prop the boat after a tune. Have you tweaked the engine height on your boat to get it as high out of the water as possible without cavitation ? this will reduce drag and often gain a bit more top end speed. Is the outboard reaching full rpm with the prop you are currently running ? I now have 7 spare props for my 115 Yam and found mercury props to be the quickest, no doubt why mercury do so well racing.
Toy could look at the Mercury 115 which is lighter and outputs more power.

[beerbelly]

i have a 115 mariner on my 5.5 destroyer and it goes really well about 38 knots and cruises comfortably at 25 at about 4k rpms .bear in mind the max transom weight is 190 kg if your 115 yamaha is a de tuned 130 it may be over the limit my mariner is a tuned 100 and is only 170 kg

[Pikey Dave]

I’m not familiar with the line up of engines in question. Is the 100hp the same block & displacement as the 115? If so, then I wouldn’t bother swapping. If the 115 is the next block size up, then there may be a case for upsizing. It depends on several factors, cost/weight/economy/faff

[69cmw]

I had a 115hp 4 stroke Yam on my 5.5 destroyer, propped correctly it used to do 48mph.
My mate has the same rib, he has a 175hp 2 stroke Yam on his, he gets about 60mph, but gets a bit lairy at that speed.