Experience of a 15hp over a 9.8hp on a 3.2m SIB?

[Max...]

Anyone here upgraded from a 9.8hp 2S to a 15hp 2S on a typical 3.2m quality air floored SIB?

Looking for real world experience of the gains.

I may have the opportunity of changing my 9.8 Tohatsu for a 15 Yam. For the record on my Zodiac 3.2m the Twatsoo tops out at 17.5 kts with one adult and one small child. Adding my (lightweight) wife and second small child it will plane with careful weight distribution and tops out at about 14.5 kts.

It would be nice to be able to add a few kts to the usual load (me and small lad) but also be able to plane with me and say two or three adults.

[Fenlander]

Your speeds are actually pretty high anyway Max if they really are kts not mph. It happens my Garmin is usually set to mph so that's what I've quoted.

I've never swapped from a 9.8 to 15 on the same inflatable but the results logged on my gps have been so consistant across the 3.1/3.4/3.5/3.6 inflatables we've owned using both 9.8 and 15 they may help you decide. My results show that between 3.1m & 3.6m the boat size makes less difference than the actual load carried.

I think we're a slightly heavier family than you so you will gain a bit over our speeds I'm sure. When I refer to self and one teen we're looking at a total of 20 stone. Two adults and two teens will be almost 40 stone plus 40lb dog, two fuel tanks, anchor and lots of "day out" kit.

Looking back at the downloaded Garmin logs I've saved the 9.8 Tohatsu would max out with self and one or two teens at 16-17mph. 14mph was our cruise. As I've said in other threads adding the 2nd adult to make four up plus kit meant 8-9mph displacement at max.

The 15hp with one adult and one teen maxes out around 23mph... add the other adult and teen plus full kit and the max is nearer 20mph. Fully loaded a 16-17mph cruise leaves a bit in hand.

I suppose the massive difference with the 15hp is that it enables planing with the full family load and pops up on the plane easily without shifting and trimming to ease it up.

I know quite a few folks manage 20hp+ outboards but for me the 9.8 was the max that was comfortable to carry about in soft sand or on a greasy slipway in beach shoes. We only lift the 15hp from car straight onto the adj boat then use the transom wheels to get it to the water. It has changed the places we use the boat but not been a problem.

Our teens at 15 & 17 are already chosing holidays with friends rather than us so it is highly likely in a few years we'll go back to an air-floor and 9.8 for the two of us plus dog.

[benc]

I upgraded from an 8hp to 15hp on my 3.2 honwave - and its made quite a bit of difference - with the 8hp, I had trouble getting 2 heavyweight (15-18stone) adults and two kids about 6 stone each on the plane... but the 15hp pops up and shoots along. I've not measured it on the GPS yet - but all being well, I will be doing it on sunday! If you have the option to goto the 15 it will be q huge difference and no faffing around balancing weight to get on the plane!

[Landlockedpirate]

I had a 15hp Yam on a Honwave 3.2, WOT with just me onboard was 22kts, WOT with 3 large adults was 20kts.

But I felt the 15hp was actually a little overpowered for the airfloor, it used to bulge when using full power. I used a 8hp Yam (2 Stroke again) and felt it performed much better, but would not plane with 3 adults on board.

[PeterM]

I think you only see the real gain when you load the boat up . For example I can do 25 knts with 50hp and 3 people as easily as a 500hp boat, but put 14 people on mine and its becomes a tad slow! While with 500horses its not noticed ! I feel sure I used to know some physics behind this .....once

Noticed this with MattH s 15hp and my 25hp. I was cruising while he was close to WOT ( but isn't he always ? eh Matt ! )

[Boats&Outboards]

Got a Yam 15 down here (Plymouth) available at the moment. Feel free to bring your boat down and give it a try

Peter @ Boatsandoutboards4sale ~ askboatsandoutboards4sale@sky.com ~ 07930 421007

[bingosucks]

get a 15 you won`t believe the difference ... i couldn`t plane 2 adults 2 kids with an old merc but the 15 mariner flys

[stork]

Hi, I upgraded my 3.2 honwave from 3.5hp 2S (Selva) to 20hp 4S (Honda) - little bit overpowered . Top speed just me onboard was 42kmph (24kts), with 4 large adults was 13kts. Towing teenage skier up to 70 kg around 16 knt. Difficult to get on the plane with skier overboard, needs about 10 sec full power and good balance on the bow (throttle extender needed), but it works... great fun cheers

Honda bf20 - test run (aton 2011) - YouTube

[guernseylee]

Originally Posted by Boats&Outboards

Got a Yam 15 down here (Plymouth) available at the moment. Feel free to bring your boat down and give it a try

Peter @ Boatsandoutboards4sale ~ askboatsandoutboards4sale@sky.com ~ 07930 421007

wana swap for 2006 df 9.9 suzuki s/s

60 hours use
was in storage for 5 years before i got it fully serviced and runs sweet

[Max...]

Thanks for all the feedback chaps, very useful (thanks for the offer Peter).

Fenlander, great info - yes, those are our true GPS measured speeds. As we are a bit lighter I'm guessing the Yam 15 might see around 25mph in that case with our usual light load which is a useful 5 mph gain.

I'm not sure how boat length affects things - I believe the extra length offsets the extra weight and is more efficient in getting up and planing etc - hence a 3.8m might be faster with the same Yam than a 3.2m? Anyone have a theory on this?

[Leapy]

Stork...nice video

[Boats&Outboards]

Originally Posted by guernseylee

wana swap for 2006 df 9.9 suzuki s/s

60 hours use
was in storage for 5 years before i got it fully serviced and runs sweet

Thanks for the offer but cannot do a straight swap. I'd have it up for sale at £995-£1250 so I may as well sell the yam.

Peter @ Boatsandoutboards4sale ~ askboatsandoutboards4sale@sky.com ~ 07930 421007

[steco1958]

Max,

I uprated my 9.9 Tohatsu many years ago, this in effect changed it from a 9.9 to a 15 Hp.

The change was a restrictor on the carb i believe.

Wondering if you have a similar device ?

[Max...]

Ahhh, no, not possible - the 9.8 and 9.9 are two different engines.

[Jonny H]

Go for the 15 hp ... my Zodiac used to fly along with my Merc 15

[Max...]

Well, I found one chaps and hope to pick it up for this weekend. Will let you all know what difference it makes.

[timrdoyle]

Hi Max, I had a similar experience with my old Zodiac 3.1. Initially had a 10hp 4 stroke and on the tidal Thames it felt slightly underpowered - struggled to get up on the plane against the tide. Upgraded to a 15hp 2 stroke and there was a noticeable increase in power, can get up on the plane against all but the strongest tides. Can't offer any speed stats, but definitely worth the expense in my experience. Tim

[ASH79]

I've just purchased a brand new 2 stroke 15hp Mercury. Previously had a 8 hp 2 stroke Mercury. 2 people with a load of fishing gear I could plane easily with my 3.4m quicksilver sport. But 3 people with same load not a chance. Could almost get up on plane but not quite. I'm testing out the 15 hp this weekend. I will report back with my results

[Flyguy]

Tohatsu 9.8 are like gold dust due to their weight. The 15 is considerably more heavy. I reckon you would be able to do a straight swap with a 15 owner looking for something lighter.

I have a 9.8 on a 3.4 an it flies.

[EnglishLes]

Originally Posted by timrdoyle

Hi Max, I had a similar experience with my old Zodiac 3.1. Initially had a 10hp 4 stroke and on the tidal Thames it felt slightly underpowered - struggled to get up on the plane against the tide. Upgraded to a 15hp 2 stroke and there was a noticeable increase in power, can get up on the plane against all but the strongest tides. Can't offer any speed stats, but definitely worth the expense in my experience. Tim

I don't understand why a (stong) tide would make a difference. As to get on the plane you surely need the same water speed whether you are going with the tide or against, assuming the water is level.