Optimum roughness on a 30 knot boat!

[ulyden]

Hi
I have a 22foot boat with a 20deg V hull! 1500kg! I have used a self polishing bottom paint.

This has now a quite uneven roughness. Today I’m doing 31 knots at maximum!

How should I treat the bottom and the paint on this? Water grinding? Special roughness?, spray on the paint or roll? Self polishing paint or hard? The boat speed is more or less the same when I take it up in the autumn! I want maximum speed to keep the engine load as low as possible! Any experience?

 

[gjgm]

I m not sure what you are asking.. about antifoul?
If the boat has a few years of age, antifoul can build up. Its a bit debatatable whether it ever builds up to such a quantity to effect the performance, but usual thinking is that its a good idea to remove the build up after some years. 5?10? On 22ft, I d suggest a big decorator's scraper.. it looks like a large windscreen squeegee with a blade on the end, and half a days scraping!
You can then apply the new antifoul of your choice. It depends where you are based, and how often you use the boat, but broad experience is that modern a/f are pretty good for UK coast, so its a bit of personal preference. Use a gloss (not emulsion) small decorator's roller, between one and two coats.
But I wouldnt worry about how smooth the hull is.

 

[Nick_H]

I'm not sure if you're saying you've lost speed, or if you've gained a knot?

I'm not one bit convinced that uneven antifoul affects boat performance, I even think there's an arguement that it may improve it (as per dimples on golf balls), though any effect either way would be so slight as to be negligible IMO. So, if you've lost speed, i'd look elsewhere for the cause.

 

[ulyden]

What I meant. I’m doing 31 knots and that’s fine! Havent lost or gained any speed! My bottom is covered with 7-9lyers with anti fouling or bottom paint! What I’m wondering about: Is there any point in removing this old paint, make the bottom smooth and use a silicone based anti fouling paint?

Could I gain some speed?

Should I spray it on?

I can se that you means that its no point but i have a neighbour!

My neighbour had a 23 foot boat. He asked me to increase the power on his engine. He had lost 5knots from 35 to 30. I measured the fuel flow to the engine and increased it to the specifications. The boost went up to 1.7 bars and he gained 1 knot! The engine was a VP 41 200hp engine with 1700running ours.

During the winter he did to things! He bought a pair of new propellers (same size) and he removed all the old paint and sprayed new on . He got back his 36 knots! I only don’t know if it was the propeller or the bottom doing the difference. I inspected the old propeller and it was in relative good shape. It vas a small 0.5ccm mark in one of the blades. Else they were perfect!

 

[Nick_H]

Your friend most likely lost the 5 or 6 knots because of fouling on the hull or propeller, which would have been cleaned by the time you saw it, and not because of uneven hull paint, or slight marks on the prop.

 

[ulyden]

My neighbour was struggling with his speed for 2 or 3 years. First winter he changed to an upgraded turbocharger and overhauled the nozzles!
I helped him with checking the diesel pump timing!He always cleaned his boat and propeller before he putted it on the water in the spring!

When we were measuring the fuel consumption I remembered that we polished the propeller (400 paper) so I don’t think it was the reason!

 

[stelican]

Hi
I have a 22foot boat with a 20deg V hull! 1500kg! I have used a self polishing bottom paint.

This has now a quite uneven roughness. Today I’m doing 31 knots at maximum!

How should I treat the bottom and the paint on this? Water grinding? Special roughness?, spray on the paint or roll? Self polishing paint or hard? The boat speed is more or less the same when I take it up in the autumn! I want maximum speed to keep the engine load as low as possible! Any experience?

I have a boat of similar size 130hp,I removed six coats of patchy antifouling and reapplied using a brush for smoothness the speed increase was more than 3 knots

 

[ulyden]

Thanks I need some motivation for removing the paint!

What about this new silicone anti fouling?

 

[Nick_H]

Ulyden, if your mate scrapes off the uneven antifoul and applies new, without making any other changes to the boat, then let us know if he gets an increase in speed. I confidently predict he doesn't, but hey i've been wrong plenty of times before.

 

[ulyden]

He got 6 knots more! He even reduced the fuel to the engine! But he also changed from an old polished propeller to a brand new! (Duoprop)

His hull had at last 10-15 layers of old paint and looked bad. He sprayed on the paint after grinding the whole underwater hull! He also had a lower V angle (10deg?) Maybe more friction?