Takes very few barnacles to affect speed

[MikeP]

A couple days ago we went down to St Michaels. The boat would not plane and top speed was 13k at 2000RPM. The engines would not rev higher. I'm used to that here (and have posted about it) and figured that the props/shafts were heavily covered with barnacles.

We anchored in St Michaels harbor and I dove the boat yesterday. There were essentially no barnacles on the prop shafts (they had been painted when the bottom was painted last year). But there were what I would call "some" barnacles on the props - really not many at all and it was hard for me to believe that those few barnacles could have made any real difference in the speed. But I scraped them off (and installed new zincs - old ones were essentially gone).

Today, on the way back - on plane at 18K!

 

[REBrueckner]

Wow!!....

good to hear you got out!

Barnacles around the hub can be somewhat tolerated, but when on the blades and especially outer reaches of blades where rotational speed/drag is greatest, big difference fast.....

 

[Pascal]

Yes amazing what just a few barnacles can cost you!

We just missed each other... We were anchored in st michaels Monday night, the Tuesday and wednesday in Annapolis!

 

[MikeP]

Pascal - sorry we missed you - we thought about going to St Michaels on Monday but decided to delay...

 

[krush]

It has something to do with drag goes up lots with speed or mumbo jumbo like that.

 

[TopHattandTails]

Mike- that's impressive that you can still get on plane and even more impressive you can turn 18 knots! Thats a whole different boat compared to mine. Kudos.

 

[MikeP]

When we did the original sea trial it actually hit 18.4 but it was very light then! It's never done that since. In fact, with a clean bottom/props/shafts it was hard pressed to hit 17 and mid 16k was pretty much normal. But last season I removed the "depropped" setup that had been on the boat when we bought it and went back to a set of unused (still wrapped in the original paper) spare oem props that came with the boat when it was new.

Now it will do 18 in calm water though it takes it probably a minute or a bit more to get there. Won't be winning any drag races - 0 to 20 in one minute is not quite the cutting edge of acceleration!

 

[REBrueckner]

MikeP....

If my '72 48 YF couldn't do 22.2 knots...according to "them"..... as I previously posted, with the same 8V71TI's as yours, then surely your heavier and wider 53MY can't do over 18 knots....according to "them".

 

[67hat34c]

conversly you put a little cup in a flat prop and what a difference. Doesnt take much to mess with prop thrust, disturb the surfact just a little and...........

Krush I expected a much more technical response from you.

 

[TopHattandTails]

Well, I'll just be happy with my 13 knot cruise and 15 knot WOT. But I will be looking into some lifting strakes for the aft and if Sky is getting good results from his extended tabs - that might add to the equation... Although I suspect, a galley and 3 heads full of granite and marble, and 300 feet of chain in the bow also have something to do with it...

 

[krush]

Krush I expected a much more technical response from you.

Only posers like to overcomplicate a simple matter....

Besides, I have a sense of humor.

 

[Boatsb]

I would expect that the area covered by the barnacles on the prop multiplied by the inverse of the shaft diameter and then divided but surface area of the rudder would give you absolutely nothing useful.

 

[the kuz]

Going under to check/clean barnacles this week. Lost about two KNTS, so we will see once clean. Water is up to 75 degrees, and I can do that.

Will let you know how much I find and how much it helps.

Regards, Bob K

 

[bobk]

Just a few barnacles or even their vestiges effect the flow of water over the props. Last fall after the starboard engine was rebuilt I had a diver clean the bottom and gear and could not get the 48MY out of the hole. We pulled the boat for a proper cleaning (and some misc stuff we were not aware of) and she hit 18 knots.

Water depth really makes a difference on these. In 12-14', it takes a minute to get to speed, in 50' it jumps right up. I generally run 8.6 knots at 1100 rpm. I got 16 yesterday at 2000-2100 in 50-60' of water in the bay.

BTW, the best fuel price I saw between Baltimore and Stuart, FL was $3.44 at Top Rack Marina south 0f the Gilmerton Bridge in VA.

Bob

 

[OBXTucker]

All this talk of 17-18 knots is making me jealous!!
Ya'll are talking DOUBLE my numbers!
Then again, I can go a long ways before fill ups!

 

[MikeP]

Yeah, 18k is nice but at 45 gal/hr or thereabouts it's not too workable as a general thing nowadays - at least for me.

OTOH, as I have posted before, our 53 just generally WORKS a lot better on plane. Of course, that's no surprise since it was designed as a planing hull NOT a displacement hull. So I have to admit that if I am restricted by the price of fuel to 9-10K, I'd much rather have a hull designed to do that.

To me the 58 LRC is the ideal boat but, like fuel for planing, they are above my pay grade! (and even if they weren't, they won't fit in my slip).

 

[REBrueckner]

I guess all this talk about prop sensitivty leads to a little insight about why a "minor" prop alignment change via computer (like propscan) can lead to improvements in speed/efficiency.

 

[the kuz]

spent an hour cleaning under the boat - Solomons harbor, MD, 73 degree water, a pleasure. Boat had been sitting for about two weeks, and was very lightly used over the winter. Props had barnacles on the hubs that needed the hack knife, and had a very few small ones on the blades which wiped off with a red scotch pad. Shafts were loaded and twice normal diameter. Had sprayed running gear with "barnacle buster" about a year ago when lifted out to change zincs and clean lots of barnacles off props/shafts. That product seemed to hold them down a little, and for $20 -$30, I'll use it again.

I'm shocked that the boat had only lost 2 or 3 KNYS at cruise. Also had quite a few on trim tads, rudders, inlets and other metal parts which needed scraping. Had small "sets" all along the edges of the underside which wiped off with the latex glove. So it is time for a haul, proper cleaning and new paint job. Blue Water Yachts ablative 67 lasted 2 1/2 years, and seems to work about as well as Micron 66 here in the mid Chesapeake. Micron 66 is the soft paint most local yards use.

Boat picked up 2 KNTS at cruise, but is still about a one under where it should be (2 at top end). Will post again in about two weeks after re-paint.