Tohatsu 50HP Prop advice needed...

[ian parkes]

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 .

[J@mes]

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.

[ian parkes]

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 .

[J@mes]

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!

[ian parkes]

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

[Poly]

Originally Posted by J@mes

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.

[J@mes]

Originally Posted by Poly

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!!

[Poly]

Originally Posted by J@mes

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.

[J@mes]

Originally Posted by Poly

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!

[J@mes]

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 rpm’s 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 it’s 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!

[J@mes]

Bump!

[J@mes]

Right!

I finally got round to buying the Solas 11 x 15 propeller to replace the OEM 11.1 x 13 Tohatsu one.

I fitted it today and unfortunately I have to say I am very gutted! =( I am getting bad cavitation, think that's what it's called where the water comes splashing up behind the transom. It never used to do that with the old prop! Also my WOT rpm with just me in the boat has now gone from 6000 rpm on the rev limiter to 5000 rpm!!!! and my max speed is down 2 mph too!

Why on earth have I lost 1000 rpm?! I was expect a reduction of 300 - 400 rpm by going up 2" in pitch (with an increase in top speed!) and also I was not expecting bad cavitation if that's what the splashing I am experiencing is called. I will do some research now but thought I'd ask you guys first! Do I need to raise my motor up one hole to stop this cavitation? TBH I don't want to do this and don't think I will be able to, if I had known this would have happened I would have simply stuck with the OEM prop! Hopefully I can get a refund if this isn't going to be a simple fix!

I appreciate your help, thanks!

[ian parkes]

At risk of being rude James we gave you the advice and help and worked out that your set up was as near as dam it as good as it can be . Put the original prop on and back off the last few hundred revs when alone in the boat . Small boats are so sensitive to load /rpm you have to compromise a bit .

[J@mes]

Ian I decided to change the prop because of the general opinion I got from this and other threads and also from speaking to boating/propeller experts on the phone.

Also something I didn't say in this thread; last time I used my rib I used a pressure gauge I had recently bought and found I was under inflating the tubes, and after inflating the tubes properly the boat was slightly faster when loaded and revving slightly more, it would rev to 5900rpm with 2 people!!! This is why I bought a bigger prop, to bring the revs down slightly! I hated the fact my engine was hitting the rev limiter and over revving. I shouldn't have to back off the last few hundred rpm.

Anyway back to topic, does anyone know what/why I am now getting this splashing?

I am with my boat for 3 more days and if I cannot sort this then the OEM prop will be going back on!

[C-NUMB]

The problem is, that if You change both make of propeller and pitch, outcome will be complete guesswork, the design differences between various brands is so big. I noted this on my tohatsu 25 and also recently on my 90 yam(same pitch, big difference in performance).

Go with the stock 11x13 or borrow another 11x13 , black max, solas, that might make the trick!

[J@mes]

Originally Posted by C-NUMB

The problem is, that if You change both make of propeller and pitch, outcome will be complete guesswork, the design differences between various brands is so big. I noted this on my tohatsu 25 and also recently on my 90 yam(same pitch, big difference in performance).

Go with the stock 11x13 or borrow another 11x13 , black max, solas, that might make the trick!

I'm surprised at what you say, but at the same time agree as it makes sense!

Also the Solas prop looks identical to my OEM one and also I have been told by a couple of different propeller specialists now that Tohatsu no longer actually make their own props and I must admit my OEM prop looks identical in how it is made and the style to the Solas prop! I have also noticed that Solas are the only prop manufactuer that sell the exactly the same diameter and pitch sized props as Tohatsu have in their prop chart! This is also a coincidence! Lastly I read somewhere on the internet that Solas make the Yamaha props and someone told me that Solas make some of the Tohatsu ones.

Either way the style of the two props I have look identical, so I am shocked I have lost 1000 rpm for a 2" pitch increase!

Also whilst I am here, I refitted the OEM prop today and I now think the splashing as I call it is not a problem as it is similar with both props, just slightly worse with the slightly higher pitched Solas prop!

As for speed, the sea today and yesterday has been flatter/calmer than normal and I have noticed before that in dead calm seas my max speed is a couple of mph less than when there is a bit of chop/waves. Well yesterday with the Solas prop I hit 38mph @ 5000 rpm (WOT) and today I hit 37mph @ what looked like virtually 6000 rpm but wasn’t quite 6000 rpm as the limiter was not kicking in (the water was dead flat this evening!) This miffed me until I remembered that my rib goes faster with some waves, only 2mph faster. I thought I would mention this as it shows that the solas prop was indeed 1mph faster at 5000 rpm than the OEM prop. I am just miffed at my WOT loosing 1000 rpm with it though!