Bottom/hull paint affects on WOT speed ?

[baz]

Subject: Bottom/hull paint affects on WOT speed ?

Ever since comparing my WOT speed of my 1yr old 2018 R27/OB that had bottom paint with another 4 month old 2018 R27/OB without bottom paint running at WOT speed I've wondered just how much performance is degraded due to bottom paint.

The comparison was (in salt water on a calm day with no wind and zero chop)

Boat #1 - My 1yr old R27/OB WOT speed with its bottom paint was at around 38.4 mph. (Yamaha 300HP outboard)
Boat #2 - The 4 month old R27/OB WOT speed without bottom paint was at around 41 mph. (Suzuki 300HP outboard)

The #2 boat was some 1/2 mile ahead of me when making this comparison and was slowly getting further and further ahead of me. I (boat #1) had no chance of keeping up with it, and no chance for over taking it. 😉

The #2 boat had two adults on board and at least 3/4 full fuel tank.
The #1 boat had one adult and a full tank of fuel.
I assume both boats had equal amount of gear stowed on board.

My boat #1 had a full cockpit enclosure and a kayak on cabin roof.
Boat #2 had no cockpit enclosure nor a kayak on top.

Ignoring the affect of the cockpit enclosure and the kayak on my boat #1 there's a difference of 2.6 mph for the identical boats except for the bottom paint on my #1 boat.

Could the bottom paint on my #1 boat explain why my WOT speed was 2.6 mph slower ? This is 6.34% slower.

If I use my boat for say 100 hours each year suffering from the 6.34% loss in speed due to the bottom paint I'm using more fuel to get to my destinations compared to boat #2 without bottom paint.

Let's work this out... at WOT speeds.

Boat #2 cruising for 100 hrs travels 4100 miles.
Boat #1 cruising for 107 hrs travels 4100 miles.

According to manufacturer's engine performance specifications both engines will burn some 26.5 GPH at WOT. Thus boat #1 would consume 7 x 26.5 = 185.5 more gallons of fuel compared to boat #2. At $4/gallon this amounts to $742.

I will assume bottom paint for the R27/OB is some $2,500 done every 3 years.

Thus for boat #1 cruising at WOT for same number of miles (4100 miles) each year as boat #2 then $742 more dollars per year is spent for boat #1, or over 3 years this will be $2,226.

It seems from this that from a fuel burn expense the bottom paint is really costing a lot of money; Over 3 yrs boat #1 has to spend $2,226 more on fuel and some $2,500 for bottom paint (a total of some $4,726 more).

I know this probably is not very precise and assumptions have been made, but I'm thinking bottom paint maybe a waste of money and simply having the boat #1's hull power washed once or twice a year is a better solution.

Comments most welcomed. Thanks... 🙂

 

[Scuba Too]

Hi Baz - looks like the other subject kicked off this new one 🙂

Bottom paint definitely takes some speed off. The surface isn't as smooth and it adds weight to the boat. However if you keep you boat in the water like I do there's not much choice as boats will get a substantial amount of growth on them over a year (which would also slow you down). If kept on a trailer I would definitely think twice about it unless extended stays in the water are routine.

 

[Chimo]

Hello Barry,

I think the growth, even slime, that will accumulate on your bottom paint is probably more of a drag than the actual paint itself. With respect, I believe your camper back and certainly your kayak will have had a lot greater effect. With the hull design like the R 27 OB there are a whole range of influence factors. You mentioned different motors. How do the weights compare? Are the propellers identical? Did either boat have tabs deployed at all? Then, perhaps, one of the most important factors, were the motors trimmed for maximum speed?

Fascinating stuff!

David

 

[Red Raven]

I used to keep my previous boat in the water through the summer months with no bottom paint here in the PNW on Saratoga Passage. The slime starts to build up after about a month. The barnacles start to grow at around 2-3 months. I had to pull the boat and pressure wash it at least once during the summer to keep it from getting severe build up. I would estimate that to keep a drag advantage over bottom paint you would have to pull the boat and pressure wash it at least every three months. So best to figure that cost (plus the hassle) in at 4 times per year. Personally I wouldn’t recommend it unless you plan to trailer your boat between each use.

 

[baz]

FYI... I don't let this sort of enquiry interfere with my boating pleasures. I simply raise the issue as my engineering background will at times cause me to wonder about such things. :lol:

As some people have mentioned, there are many things that affect boat performance and it's very difficult to get a precise Apple-to-Apple comparison.

Some have indicated my Cockpit enclosure actually offers better aerodynamic efficiency/performance compared to having an open cockpit, along with not really knowing engine and prop differences, and so on. But as I said, I've made some pretty broad assumptions in my OP which simply have resulted in 'opening a can of worms'. 😉

 

[ixlr8]

Subject: Bottom/hull paint affects on WOT speed ?

It seems from this that from a fuel burn expense the bottom paint is really costing a lot of money; Over 3 yrs boat #1 has to spend $2,226 more on fuel and some $2,500 for bottom paint (a total of some $4,726 more).

I know this probably is not very precise and assumptions have been made, but I'm thinking bottom paint maybe a waste of money and simply having the boat #1's hull power washed once or twice a year is a better solution.

Comments most welcomed. Thanks... 🙂

Barry,
NO NO NO... My boat was in the water for 7 weeks without bottom paint. My top speed went from 41mph to 28mph due to growth on the bottom in that short amount of time. At least in Maine waters, which never get above 55dF, I would have to pressure wash my hull every 2 weeks to be able to maintain a 40mph top speed.

 

[baz]

Jim: Thanks.... good to know the expense for bottom paint is a worthwhile thing to have. 😀

 

[JamesTXSD]

All things being equal, and in this circumstance, they certainly weren't, the weight difference with bottom paint vs without may be about the same as a 1/4 full holding tank vs 3/4 full tank. Even with bottom paint, you have to regularly clean your bottom. The bottom paint means the slime will come off easier and there is less opportunity for barnacles to adhere. If you hadn't had your bottom cleaned in a month or so, that could easily contribute to the small speed difference. Weight and loading. Most efficient trim for the speed/RPM.

A full fuel tank vs partially full will also make a difference. When cruising, we would top off fuel when convenient. Around our home waters, I rarely ran the boat with even 1/2 fuel for day tripping... if you are returning home after each trip, there is no point in carrying around the extra fuel weight. It all adds up.

38.4 mph is a pretty fine speed for a comfortable boat like the R-27OB. That said, how often are owners traveling at WOT??

Maybe the point was to have a discussion, but I don't see any way that you can conclude that your bottom paint was the reason for the speed difference. Also for discussion sake, we used to have a Corsair trimaran sailboat that we raced. It also lived at the dock by our home, so it was bottom painted. I had the boatyard that did the work spray multiple thinner coats of bottom paint to keep the surface smooth. Even if you started with a textured coat of bottom paint, it won't take long for an ablative type of paint to smooth out some as you run at speed. In our warm waters of the Gulf, slime will build up in a week; barnacles will start to adhere in 10 days to two weeks... yes, way faster than the cold waters of the PNW. But if you are going to keep a boat in the water, bottom paint is a must unless you plan to pull the boat and pressure wash it VERY regularly. (And still have the bottom scraped regularly if the boat isn't run frequently.)

 

[Phil Nersessian]

I can't say I noticed a difference in speed after I painted the bottom of my boat. The weight of the 3 coats of ablative paint cured is probably a maximum of 50 pounds, not much on a boat that weighs about 12,000 pounds loaded. I think the deciding factor though is if the boat is stored in water or out. If you trailer or keep the boat on a lift I wouldn't paint, just wash after using. If you store the boat in the water I don't think you'll get away without painting. I'm in the ablative camp myself.

 

[Kim and Ed]

When asked about a glass of water, the lawyer said it was half full, the accountant said it was half empty, the engineer replied the glass was twice as big as it needed to be! Soon we will all be out enjoying our tugs.
Ed/

 

[BB marine]

If you are leaving the boat in the water the minimal "speed" Loss is not a reason for not painting the boats bottom. The first year of owning C26 we did not paint the bottom. I dry stored the boat on the trailer. We launched every Friday and put the boat back on the trailer every Sunday and stored it. I liked the look of the shiny bottom. At the end of the season I decided to paint the bottom so I could leave it in the water and not worry about bottom growth( Fresh Water). I went a step further then the factory bottom painting and applied 3 coats of Pettit Epoxy Barrier Primer and 2 coats of Vivid black bottom paint. Vivid is a hard ablative finish. I was careful making sure to keep the finish surface as smooth as possible. I used a tight nap roller pad and two thin coats just enough to cover. The best way to apply it is to spray it but I could not do that. (No spray booth) I have been told 3 to 5 percent loss in speed with applied bottom paint comparing apples to apples. 40 mph boat= 1.2 to 2 mph, 20 mph= .6 to 1 mph. As already stated many other things can change speed more then bottom paint. 1) no bottom paint with growth 2) additional weight- gear, fuel water, passengers and the distribution of the weight 3) water- Salt, ,brackish, fresh 4) trim angle- to much, to little 4) propping 5) engine maintenance and engine running performance based on ambient air and water temperatures. I personally would not question an application of bottom paint on a 29.

 

[Dcsqatar]

I am new to Ranger/Cutwater and inboard boating (waiting on delivery of a 29S) but I have a lot of experience with big outboards. As Chimo (David) noted, engine trim and trim tab position can easily account for a 5-10% speed difference between otherwise identical boats.

 

[Kevinandcarol]

Anti fouling doesn't have to be a drag. I used a ceramic coating on my R23 it protects the blue hull from fading and growth will not stick to it.Ceramic Pro has been marine tested and certified. You can find info on this product at ceramicpro.com. this made my boat faster and the water beads off, I did the entire boat with it and it really shines. I used it on the front cap of my 5th wheel and the bugs wash right off too.