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Old 29 May 2013, 19:46   #21
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My mate has my old 4.1 rib we both have identical tohatsu 50's and both boats have very similar performance . Tried the higher pitch on both boats and got same result .
Yes 2-3 mph more top end with adult and kid in boat , add another person and performance isn't as good

the small ribs are very sensitive to load , i get about 5800 with about 25 stone in the boat but add 3 or 4 bodies and some kit and it drops to about 5100-5300.
I would doubt your boat is any lighter than ours , my current alloy hull boat wieghs 105 kg and the old one was about the same or less .
I haven't tried it alone flat out alone, have always had another small adult on board . I think it would probably rev to about 6k with just me aboard and get another 3 mph maybe
What i really notice is the tohatsu has a nice sweet spot just under 4k revs for cruising where its quiet and economical . I think the timing advances after that and it starts to snarl a bit over 4k and uses more juice . with the higher pitch prop cruising at that sweet spot was not so easy the engine felt like it was labouring a little and it certainly used more fuel .
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Old 30 May 2013, 05:04   #22
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Hi Ian, another great reply, thanks!

I think I am swaying towards staying with the standard 11.1 x 13 prop now. I assume your Tohatsu 50 is the TLDI model and not the older carb model?!

The thing is I would say I never really have a heavy load, would guestimate 40% of the time is just me in the boat, 40% two people and 20% three people (+ anchor/crap when three people!)

When 2 up (me and my dad weigh 170kg/26.5 stone) WOT is 5800rpm from memory, which is about the same rpm as you then with 25 stone.

As for the Tohatsu sweet spot, yeah I know what you mean, I enjoy cruising at 3000 - 4000 rpm depending on load!

I think I will wait and double check the WOT rpm with different loads to make sure my memory is correct! Though I will keep my eye open in the mean time, if I saw a second hand 11 x 15 prop come up it may be worth a try for reasonable money. TBH I'm begrudged as it is to pay 120 for a brand new one if I will only gain 3mph top end and loose flexibility when loaded up. Sounds like I'm talking myself out of it! lol. Thanks for your advice.
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Old 30 May 2013, 13:59   #23
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your figures sound right compared to mine , but i always have to recheck . I think you are thinking the right way

I think the cheapest way to experiment is to buy another original prop from steamlined propellers trades as Razzlin on ebay very helpful and sells them for a very good price . then get it re pitched just half of what you were thinking of .

My motor is the old carb model the most basic pre mix pull start model . I have fitted boyesen reeds that seem to improve it a bit . Plus if you are likely to over rev the engine the boyesen reeds are a good safety feature .
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Old 30 May 2013, 14:28   #24
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I didn't think of re pitching, that's a good idea thanks!

Arrrrr you have the older carb model, they are meant to be very good!! I just hope mine lives up to the rep they have! Unfortunately I cant tune mine with performance reeds like you have because its injection, so yours is prob a little more powerful than mine! Though mine is quite fuel/oil efficient, in fact it doesn't use much more petrol/oil compared to my old Yamaha 25HP carb'd outboard!
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Old 30 May 2013, 16:05   #25
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its simple which is what i wanted , a bit raw at times but keep the revs below 4k and its very good on juice , start pushing it hard and about 1/2 litre a mile is about what it uses . For tiller steer there is not really much other choice , exept a honda 50 for about 6k
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Old 30 May 2013, 19:30   #26
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Well I think WOT is 6k rpm from memory with just me in the boat = 39mph.

Also from memory with my dad in the boat (+ 85kg) the top speed is 35mph and I think the WOT is slightly less, maybe 5800 rpm.

What advice can you give from this?

Also just noticed on the Tohatsu chart is also states the stock prop 11.1 x 13 is for a boat with a moderate weight load (500-1500 lb.) but it says for a light weight load (100-500 lb.) then a 11.1 x 14 or 11 x 15 prop is recommended. Well according to this I only ever have a light weight load!! I appreciate your comments re every boat is different, but with their loading advice in mind and the fact my rib is not a heavy rib (116kg bare boat + 98kg engine + weight of battery/consol/steering/fuel etc I would conservatively guess 300kg max all in) I would have therefore thought an 11 x 15 prop would be ideal for my rib?!

Cheers,
James
300 kg = 660 lbs so you are in the moderate end (albeit the low side of it).

ABSOLUTE maximum you could expect - and only then if for some reason the prop gods were looking down and feeling sorry for you would be 6 mph increase. More likely it will be about 3-4 mph lightly loaded, and less when loaded.
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Old 31 May 2013, 04:46   #27
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300 kg = 660 lbs so you are in the moderate end (albeit the low side of it).

ABSOLUTE maximum you could expect - and only then if for some reason the prop gods were looking down and feeling sorry for you would be 6 mph increase. More likely it will be about 3-4 mph lightly loaded, and less when loaded.

You have read it wrong mate, the weight load is the exactly that, the weight load, not the weight of the boat! Otherwise it is recommending a 50HP engine for a very light boat weighing only 100lbs (45kg) and also the 140HP engine for that matter!!!

Yeah I would imagine I would get about 3mph increase, so about 10% extra top speed, but from what Ian Parkes said it looks like the cons outweigh the pros!!
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Old 31 May 2013, 08:19   #28
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You have read it wrong mate, the weight load is the exactly that, the weight load, not the weight of the boat! Otherwise it is recommending a 50HP engine for a very light boat weighing only 100lbs (45kg) and also the 140HP engine for that matter!!!
I don't think I have! They may have written it wrong! The total weight of the boat is the factor that matters (together with its hull shape which they have ignored) not whether the weight is fibreglass, steel or people.
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Old 31 May 2013, 09:31   #29
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I don't think I have! They may have written it wrong! The total weight of the boat is the factor that matters (together with its hull shape which they have ignored) not whether the weight is fibreglass, steel or people.
I appreciate and agree with what your saying, but I'm just going by what Tohatsu say!... Tohatsu Outboards: Authorized North American Distributor for Tohatsu Outboards, Parts, & Accessories.

I think the point they are tying to make is just regarding props for the "average" boat if there was such a thing! lol. As in having a constant heavy load = lower pitch prop, and a light load = higher pitch prop etc. Also they probably assume an appropriate HP Tohatsu engine has also been fitted to the boat and is fit for purpose!
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Old 12 July 2013, 05:19   #30
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WOT rpm checked!...

Right thought I would come back here. I had decided to not bother changing props and stick with the stock 11.1 x 13 one, but after double checking my WOT rpms I was a little shocked at my findings!
First of all that high pitch noise which I could often hear at WOT with just me in the boat IS the rev limiter! With the wind and engine noise it was hard to make out but when I put my head close to the throttle/control box I realised it was making a constant beep noise when I am at 6000 rpm at WOT. I looked in my manual and it says the rev limiter kicks in at approx 6000 rpm and emits a constant beep noise! I have also checked the WOT rpm with my dad and I in the boat and its 5800 rpm, and also with my mum, dad and myself the WOT is 5500 rpm.

So after double checking my WOT rpm's with different loads, I think I definitely need to change my prop for a bigger pitch just for the sake of my engine as its over revving with just me in the boat! The manual says WOT should be 5150 5850 rpm. I can go to a 11.1 x 14 or 11 x 15 according to Tohatsu. I think I would prefer the 11 x 15 as it will give me a better top speed which I prefer to acceleration.

How much rpm do you loose by going up 1 in pitch though? I think someone said before it's 100 rpm, in which case with an 11 x 15 prop and 3 up in my boat WOT should be about 5300 rpm which is within the recommended WOT range.

I can get the OEM Tohatsu 11 x 15 prop for 165 delivered, a Solas 11 x 15 for 120 delivered or a Vortex 10 x 15 for 95 delivered.

Unfortunately price is a factor in my decision and therefore I would go for the Solas over the OEM as its the correct size and after a bit of research I have only found good things about Solas. What about this Vortex one though? I am put off by the fact it is 10 diameter instead of 11. What difference would this make? Would it help the engine rev ever so slightly easier due to the slightly smaller diameter? If so maybe this would be a good thing, though that said Tohatsu recommend 11 x 15, not 10 x 15!

Thanks for your help!
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