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Old 13 October 2013, 17:51   #1
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HELP, New Prop is Causing Splashing!...

I have another thread here (Tohatsu 50HP Prop advice needed...) but thought I make a new one because I need an answer ASAP as I am only with my boat for 3 more days and also because my original thread is an old one and I doubt new people will look at it because its a few pages long!

Anyway...

I finally got round to buying a Solas 11 x 15 propeller to replace the OEM 11.1 x 13 Tohatsu one which at WOT would hit the rev limiter at 6000 rpm!

I fitted it today and unfortunately I have to say I am very gutted! =( I am getting bad cavitation, I 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 about 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 have just done about 2 hours research but I am still none the wiser on whether this is cavitation or not! Do I need to raise my motor up one hole to stop this splashing problem? 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! Though thinking about it surely I shouldn't have to change my engine height as it was absolutely fine before, no splash at all, all I have done is changed the 13" pitch prop for a 15" pitch one!!!

Lastly I also noticed the exhaust note is slightly different now with the new prop. Thought I'd say in case that's a clue to the problem as it's a through hub exhaust prop!

I appreciate your help, thanks everyone!
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Old 14 October 2013, 01:41   #2
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Splashing is not cavitation. Most likely a motor too deep, but there could be other causes.

Cavitation is caused by water flow over the back of the prop separating from the metal to the point where the liquid vaporizes forming bubbles. Not a problem in an of itself, but when the bubbles collapse, they create a shock wave that can erode metal. Doesn't usually happen with modern props. What people call cavitation these days is usually aeration, which is air being drawn into the prop, which causes a severe reduction in "grip", over-revving of the engine, and is felt as a loss of power.

Was your old prop in good shape? Are you sure it still matched the original specs? You can fairly easily measure the diameter (measure from one blade to the near side of the hub, then the far side of the hub, add the two together.) Pitch is a little tougher.

Assuming similar blade shapes and diameters (which you report as nominally pretty similar), you are right; 200-500 delta is what I'd expect. Is the new prop stainless vs an old aluminum? You'll lose another couple of hundred from the weight of the SS, and cupping could account for a couple more.

jky
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Old 14 October 2013, 08:45   #3
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You say 11.1 diameter on the OEM prop to 11" on the Solas. Normally a reduction in diameter would increase your RPM (and it;s anything but linear), but I think this Solas prop is probably more different then you can see by eye. - Could you get an exchange?


To 200rpm/inch rule only works "properly" with similar props. It's a multi facetted balancing act you have there, which also depends on the engine's torque.


I went form 10.25/14" to 11/14" and dropped my RPM by about 1200. (admittedly I had the Yam prop lying i nthe garage, and have no fancy rev. limiter, so got a snapshjpt of just how much I needed to move & thought I;d give it a try). I much prefer the grip with the Yam 14" - the old Merc one let go at the slightest provovation - and so I'm now lookign for a 13 spline 11" diameter 13" pitch prop.

If the Tohatsu one has 13 splines I could be interested in it when you finally sort yours out......
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Old 14 October 2013, 09:51   #4
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Quote:
Originally Posted by jyasaki View Post
Splashing is not cavitation. Most likely a motor too deep, but there could be other causes.

Cavitation is caused by water flow over the back of the prop separating from the metal to the point where the liquid vaporizes forming bubbles. Not a problem in an of itself, but when the bubbles collapse, they create a shock wave that can erode metal. Doesn't usually happen with modern props. What people call cavitation these days is usually aeration, which is air being drawn into the prop, which causes a severe reduction in "grip", over-revving of the engine, and is felt as a loss of power.

Was your old prop in good shape? Are you sure it still matched the original specs? You can fairly easily measure the diameter (measure from one blade to the near side of the hub, then the far side of the hub, add the two together.) Pitch is a little tougher.

Assuming similar blade shapes and diameters (which you report as nominally pretty similar), you are right; 200-500 delta is what I'd expect. Is the new prop stainless vs an old aluminum? You'll lose another couple of hundred from the weight of the SS, and cupping could account for a couple more.

jky
Yes my old prop is/was in VERY good shape, my rib is only just over a year old and not used much! Tohatsu prop chart says after the 11.1 x 13 OEM prop I can go to a 11.1 x 14 and finally a 11 (not 11.1!) x 15 prop. All these props are aluminium and the I chose Solas because their sizes (unlike other manufactures) are identical to the Tohatsu ones and also the design is. Both props look very very similar in design!!



Quote:
Originally Posted by 9D280 View Post
You say 11.1 diameter on the OEM prop to 11" on the Solas. Normally a reduction in diameter would increase your RPM (and it;s anything but linear), but I think this Solas prop is probably more different then you can see by eye. - Could you get an exchange?


To 200rpm/inch rule only works "properly" with similar props. It's a multi facetted balancing act you have there, which also depends on the engine's torque.


I went form 10.25/14" to 11/14" and dropped my RPM by about 1200. (admittedly I had the Yam prop lying i nthe garage, and have no fancy rev. limiter, so got a snapshjpt of just how much I needed to move & thought I;d give it a try). I much prefer the grip with the Yam 14" - the old Merc one let go at the slightest provovation - and so I'm now lookign for a 13 spline 11" diameter 13" pitch prop.

If the Tohatsu one has 13 splines I could be interested in it when you finally sort yours out......
As said above Tohatsu do the 13”/14” pitch props with 11.1” diameter but they use an 11” diameter for the 15” pitch prop!



Does anyone know why I am getting all this splashing? It's not like the motor height has changed or anything else!
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Old 14 October 2013, 12:46   #5
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Where is it coming from? - Straight up the front of the leg, out at 45 degrees / something else?

Assuming the trim / weight distribution of the boat / etc is identical, I would have thought loosing 2 knots (or whatever it was) would reduce the splashing, BUT the small drop in speed might have just lowered your cav plate into the full brunt of the water shooting out from under your hull.

Have you played with the trim? Apologies if you know about this already, but you are trimming out 'till it lets go then notching it back in slightly? An engine too far in will cause a nice "Bellagio effect".... and you will hear the note change / steering will become lighter & it generally sound happier.

If that doesn't work, have you got a friend who could hold a waterproof camera over your transom or tail you in another boat & get some pics? If we knew what was causing it you're well on the way to a solution.....
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Old 14 October 2013, 16:47   #6
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Quote:
Originally Posted by 9D280 View Post
Where is it coming from? - Straight up the front of the leg, out at 45 degrees / something else?

Assuming the trim / weight distribution of the boat / etc is identical, I would have thought loosing 2 knots (or whatever it was) would reduce the splashing, BUT the small drop in speed might have just lowered your cav plate into the full brunt of the water shooting out from under your hull.

Have you played with the trim? Apologies if you know about this already, but you are trimming out 'till it lets go then notching it back in slightly? An engine too far in will cause a nice "Bellagio effect".... and you will hear the note change / steering will become lighter & it generally sound happier.

If that doesn't work, have you got a friend who could hold a waterproof camera over your transom or tail you in another boat & get some pics? If we knew what was causing it you're well on the way to a solution.....
Right this is a bit embarrassing, I put the OEM 11.1 x 13 prop back on today and sure enough there is splash coming out of the back, but not quite as bad. I obviously never realised it before, but the 11 x 15 Solas prop definitely produces slightly more splash. Would you expect a higher pitched prop to do this? When I say splash I mean that water comes out the back of the boat/transom into the air, I know that sounds very vague. I did look at the engine leg yesterday with the 15” Solas prop on and the water was coming out horizontally from all around the cavitation plate I think from memory and also the thing that I noticed the most which I obviously didn’t notice before with the OEM prop was that due to the shape of the hull, some of the water that comes out from under the hull shoots up at about 45 degrees away from the boat, and I think this is slightly more aggressive with the solas prop. Also I had never really noticed just how much water/spray comes out from under the hull/the back of the boat when going along at WOT, reason being I am concentrating on where I am going at WOT!

Does my splashing description sound normal?

The trim and weight distribution of the boat is identical, though I did play with the trim for the solas prop and it did help slightly trimming out about 25% on the gauge.

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. However I am disappointed that going from 11.1 x 13 to 11 x 15 that I have lost 1000 rpm at WOT!!! I was expecting a loss of about 300 – 500 rpm, not 1000rpm! Why do you think this has happened? In hind sight maybe I should have I got a 14” prop or even a 13.5” pitch one?!

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!

Thanks for your help/reading!
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Old 14 October 2013, 18:24   #7
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Hi J@mes,

Check these charts, if you want your current Tohatsu 50 HP to work as a champ, you need to have water flow skiming under small upper water deflector plate, as first example on second chart, that's Tohatsu's sweeet transom height ideal match.

As a starter add wooden shims accordingly untill this height is achieved, then go for a permanent fix including playng with the prop pitch of your preference.- Are you experimenting out or over transom water splahes ?
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Old 14 October 2013, 19:15   #8
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Hi J@mes,

Check these charts, if you want your current Tohatsu 50 HP to work as a champ, you need to have water flow skiming under small upper water deflector plate, as first example on second chart, that's Tohatsu's sweeet transom height ideal match.

As a starter add wooden shims accordingly untill this height is achieved, then go for a permanent fix including playng with the prop pitch of your preference.- Are you experimenting out or over transom water splahes ?
Hi Locozodiac,

Thanks for the reply. Will double check my set up to the pic you have shown but I am pretty sure that's exactly how mine is set up, and I should certainly hope it is as it's a Tohatsu engine on a Tohatsu rib set up at the Tohatsu UK headquarters!! EDIT: Have just looked at some pics of my rib on my lap top and the engine is mounted just like it is in the pic you attached!

In answer to your question, the water does not splash over the transom, it splashes away from the boat!


Back to rpm, why on earth do you think I lost 1000 rpm for a 2" increase in pitch?!
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Old 14 October 2013, 19:46   #9
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In answer to your question, the water does not splash over the transom, it splashes away from the boat!
That's what we call a rooster tail.

I suspect your engine's set a bit too high or the trim need to be brought down a notch.
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Old 15 October 2013, 02:33   #10
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Hi Locozodiac,
EDIT: Have just looked at some pics of my rib on my lap top and the engine is mounted just like it is in the pic you attached!
If you engine is sitting as pictured AKA with upper deflector plate even or just above bottom of the keel, then it's way too low contributing to bad performance.
If you read simple document that Tohatsu provides to you. Note position of upper deflector plat relative to keel

Subnote
Quote:
If the height difference exceeds 0~~30 mm. engine power performance is likely to be reduced as as result of increased water resistance to the gear case assembly
Quote:
Back to rpm, why on earth do you think I lost 1000 rpm for a 2" increase in pitch?!
Quite simple math. Changing pitch by 2" from 13 to 15 is equivalent to 15% reduction in gear ratio. so if you able to maintain approximately same speed as before you will observe 6000 *0.15 ~ 900 rpm drop. If your motor is too deep causing loss of performance fighting unnecessary parasitic drag from your gear case, you may observe speed drop, hence even higher drop in RPM.
Quite simply your motor doesn't have enough power to overcome it.
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