HELP, New Prop is Causing Splashing!...

[J@mes]

I have another thread here (http://forums.iboats.com/prop-questions-topics/tohatsu-50hp-prop-advice-needed-605893-2.html) 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 couple of 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!

 

[steelespike]

Re: HELP, New Prop is Causing Splashing!...

If this is on an inflatable I would suspect the boat may have developed a problem.
Is the pressure right.
Check the motor height.Is the transom solid and in the correct position.
The Solas props have a good reputation for good predictable results.
If the old prop was a plain design the Solas might produce a larger rpm change.
If the boat has developed a problem it could result in splash,slower speed and rpm loss.

 

[J@mes]

Re: HELP, New Prop is Causing Splashing!...

Yes this is a rib and it is only just over a year old and not used that much!
The pressure is spot on, I used the gauge to check it yesterday before testing the new prop!
The motor height has not changed, I have simply changed the 11.1 x 13 OEM prop for the bigger 11 x 15 one which is the size Tohatsu recommend if needed. The design of the Solas and Tohatsu prop look very very similar! Also both props are aluminium.
The boat is in like new condition, no problems!

 

[steelespike]

Re: HELP, New Prop is Causing Splashing!...

Describe the spray.
Does it come up the outside of the transom?
What is the gear ratio,Do you know the gps speed at 6000 with the original prop?

 

[J@mes]

Re: HELP, New Prop is Causing Splashing!...

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?!

The gear ratio is 1.85. As for gps 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 62kmh/38mph @ 5000 rpm (WOT) and today I hit 60kmh/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!

 

[limitout]

Re: HELP, New Prop is Causing Splashing!...

you may need to go down to a 10 or 10.5 wheel to get the rpms back up then resize the pitch correctly

props can be tricky and its not always as easy as just changing the pitch alone although most often that's all you do need to do.

I think you need to start over from scratch and get your motor adjusted to the right height and trim before you start messing with the prop.

as to the thread repost I think if you are going to do this then you need to repost all the previous data you've given so we are all up to speed and on the same page as to what you have or have not checked or done and what you have ruled out already so we don't have to read through an old several page thread and come back here to continue reading again

 

[steelespike]

Re: HELP, New Prop is Causing Splashing!...

The spray may be a characteristic of the hull.
Are the tubes tapered, rather than squared off like a typical boat hull.
Both props are doing very well with the 13 at about 7% slip and the 15 at about 10%
The lower rpm probably accounts for the slightly higher slip of the 15.
A little chop will free up lots of boats more the a glass like surface.
If the bottom is inflated rather than ridged It may need a little chop.

 

[jestor68]

Re: HELP, New Prop is Causing Splashing!...

Well, let's see.

If a 13 is too little and a 15 is too much; how about a 14"?

 

[J@mes]

Re: HELP, New Prop is Causing Splashing!...

The spray may be a characteristic of the hull.
Are the tubes tapered, rather than squared off like a typical boat hull.
Both props are doing very well with the 13 at about 7% slip and the 15 at about 10%
The lower rpm probably accounts for the slightly higher slip of the 15.
A little chop will free up lots of boats more the a glass like surface.
If the bottom is inflated rather than ridged It may need a little chop.

Yeah I think the spray is the characteristic of the hull.
The tubes are rounded off at the end, not tapped into a cone shape like many ribs can be.
It's a rib, so rigid hull.


On another boating forum someone has said the reason I have lost about 1000rpm is... "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..."

This makes sense! However I did a lot of research on whether to get a 14 or 15" pitch prop and decided to go for the 15 because I was told a 1" change in pitch makes between 100 - 200 rpm difference, so I was expecting a drop of about 400 rpm NOT 1000 rpm!!!

This rule of thumb that everyone uses seems absolutely pointless in my case as it didn't work, but the simple math equation is virtually spot! I don't understand! Why did the rule of thumb not work for me?! :facepalm:

Well, let's see.
If a 13 is too little and a 15 is too much; how about a 14"?

Using maths I think 13.5 would be ideal!! a 14" pitch would make my WOT about 5500 rpm, where as 13.5 would be 5750 rpm.

 

[limitout]

Re: HELP, New Prop is Causing Splashing!...

Why did the rule of thumb not work for me?! :facepalm:

well your hull isn't typical or standard when compared to the average boat so why would you expect "rule of thumb" calculations to be all that accurate for your boat?

I understand your frustration but "generalized" rules or guestimates are just that guestimates

 

[J@mes]

Re: HELP, New Prop is Causing Splashing!...

well your hull isn't typical or standard when compared to the average boat so why would you expect "rule of thumb" calculations to be all that accurate for your boat?

I understand your frustration but "generalized" rules or guestimates are just that guestimates

Well I appreciate that but I did explain my set up to everyone when asking about how much rpm do you loose/gain for every change in inch of pitch, but no one said any different regarding my rib! All I could do is ask, and trust me I spent ages researching (about 3 months) before biting the bullet and buying a new prop! Oh well live and learn!