Trim Tabs

[Guest]

Does anyone recommend the fitting of trim tabs on a semi displacement cruiser, (37ft 10ton) or indeed
do they achieve an improvement in knots by bringing the bow down, and consequently in fuel consumption?
Have read alot about Bennets trim tabs, however no point in the expense if they do not acheive results
on semi dis hulls. If Falcon71 is reading has your 42 CL got them ?


cneighbour

 

[kimhollamby]

What boat/speed?

Associate Publisher ybw.com websites kim_hollamby@ipcmedia.com

 

[Guest]

Hi Kim shes a Broom 37 Crown with twin 150hp Perkins turbos, cruises at about 14-15 knots,
can do 16-17 at full revs with abit of tide. Has a pretty nose up attitute at that.
As I say semi dis with big big keel.

cneighbour

 

[longjohnsilver]

Thought abouit fitting them to my 32' semi displacement boat as most others have them. Then I spoke to the designer and he poo pooed the idea saying the hull was designed not to need them.

I presume he was right, but still wonder.

 

[hlb]

They dont seem to make all that much difference with anything, yes there nice to have and may gain a knot or so.
Help a little in rough seas, but its a bit like Soltron, very hard to tell.

Haydn

 

[kimhollamby]

General rule, semi-displacement hulls with enough power for speed in mid teens might squeeze a bit more speed out of tabs, a couple of knots perhaps. There may be some Crown owners reading this who can give specific feedback.

Another general rule of thumb for semi displacement boats with big keels (wide as well as deep on Crown) and trim tabs...wind all the tab right off when running downwind, otherwise you'll find the boat becomes a real handful, to put it mildly.

Associate Publisher ybw.com websites kim_hollamby@ipcmedia.com

 

[hlb]

AH1
But thats far more true of planing boats, darnt go with following sea and trim tabs hard down,. Suisidal! But lift them up and all ok. Like being on a ski run. Past a battle ship once. ( No pun intended). F6 sea behind, tabs all the way up, flat out, safe as houses.


Haydn

 

[byron]

The Crown/Ocean/Continental all used the same hull built by Aquafibre.
Fitting trim tabs just ain't worth the expense. If you are getting these speeds alreay then believe me you are at the top end. In fact, are you sure they are 145s and not 175s as 17 knots is pretty impressive on a Crown.

ô¿ô

 

[kimhollamby]

Yes, it's the right advice for all boats but have you tried trim tabs hard down on a big keel semi-displacement going flat out downwind? Only for adrenalin junkies!

Associate Publisher ybw.com websites kim_hollamby@ipcmedia.com

 

[coliholic]

So what happens then Kim? Serious Q. Does it tend to wander or dig in the bow or what?

 

[kimhollamby]

In short, the boat will try to swing around the heading both ways or even broach...swinging uncontrollably sideways to the waves with all that entails. If you have an inexperienced helmsman who over-compensates then it adds to the mix.

Couple of things happen in combination. Too much lift at the stern and added push of following wave reduces the effectiveness of the rudders and digs the bow in so the only thing the boat doesn't want to do is go straight. It doesn't entirely help if the boat is on the way back from France loaded with 10 crates of Christmas festivity in the forward cabin!

Even without tabs, some of the older motorboat designs that had big keels with broad flat bases (that themselves helped to generate lift) can be a bit of a handful if fitted with large engines and pushed hard downwind.

There's good news in all of this for virtually any of this type of boat; tabs up, slow down a bit and the symptoms reduce to manageable levels or disappear.

Associate Publisher ybw.com websites kim_hollamby@ipcmedia.com

 

[Guest]

Re: KIM Trim Tabs

Hear what your saying about the wide keel which they tend to plane on, but my point about the bow
high attitude in head on seas, would not the trim tabs enable more of the hull to be in water and stop the
thing from climbing the hill as it were. Surely this would save fuel?

cneighbour

 

[hlb]

Re: KIM Trim Tabs

Your right about keeping nose down for going into waves. The bows will not lift as much and better ride. But cant see it saving much fuel. Thing is you will have to save an awfull lot of fuel for the cost involved. But if money no object go ahead. Its got to be better. Question is. How much.

Haydn

 

[kimhollamby]

Re: KIM Trim Tabs

Head seas entirely the opposite and tabs may well help your case there by allowing the nose to do a bit more of the work, rather than the underside of the boat. That said, I agree with Byron that if you are getting 17 knots actual out of a Crown then you are probably doing quite well already.

Would tabs save fuel...well there is a case for saying yes if you can get the slightly better speed for the same throttle setting. And the likelyhood is you might. Completely unscientific but let's say 1-2 knots at mid teens speeds...interesting in fact that 1 knot extra at 15 knots equals a 7 percent saving, assuming that you get all of that gain from the same fuel burn (ie that the engines don't consequently burn more fuel at a higher loading for the same rpm).

Would the saving justify the expenditure. Maybe not, but if you were keen to have more control over the boat's running angle anyway then justification is a bit easier. And heh, it's a new toy (delete last sentence if forwarding any of this as evidence to budget board).

Associate Publisher ybw.com websites kim_hollamby@ipcmedia.com<P ID="edit"><FONT SIZE=-1>Edited by kimhollamby on Fri Jan 25 16:18:00 2002 (server time).</FONT></P>

 

[Falcon71]

Too technical for me, but a quick phone call later!! No we dont have trim tabs,we don't need them, we have a cruising trim angle of 5 degrees. Hope that helps.

 

[Guest]

Every hull has it's maximum design speed ,so for a semi displacement hull once you have got to the maximum design speed putting more throttle on will just increase your bow wave with just a slight increase of speed but the fuel figures will become a monster. This is termed as the hump most hull's can get to the hump but only planning hulls go past it .
Tabs were meant for planning hulls to help the hull get on the plane quickly through the hump stage thus enabling the throttle to be eased back and fuel savings.Your designer is right .
Mick

http://homepage.ntlworld.com/boats
looking for an ex trawler if you see one !!

 

[kimhollamby]

You're talking much more about a displacement hull here. Shovel huge amounts of power in a genuine displacement hull and it goes nowhere. Shovel huge amounts of power in a genuine semi-displacement design (which often has no discernible hump speed) and you will tend to go faster, albeit very inefficiently and sometimes with very undesirable results!

Associate Publisher ybw.com websites kim_hollamby@ipcmedia.com

 

[miket]

Speak to the horses mouth.

Ring Andrew Stone at Broom. He will tell you what effect they have.

We have a Crown in our club, currently out of the water, and I am pretty sure it has a fixed transom extension. (full width of transom. Have travelled with them at speed and 17/18 kts was about it and it looked pretty nose high.