Interceptors vs Tabs

[Willallison]

I'm interested to know about the relative efficiency and effectiveness of Interceptors (such as the QL from Volvo) vs traditional trim tabs at lower planing speeds. By lower, I mean in the 10 - 20 knot range.
Any experiences?

[Ad Hoc]

They both need speed to work...without a high water velocity, the 'lift' or trimming moment is very little to non-existent.

[Willallison]

Yes - of course - but which works better at lower speeds is what I want to know. Instinctively I would think the tabs would, as they are not reliant on speed to create the angled 'surface area', but I have no data or experience with the interceptors to back that up....

[Ad Hoc]

Hmmm.. this is not an easy one to answer in terms of which is better at lower speeds, since there are too many variables. For example, the type of propulsion train will affect the 'suitability' of either. A twin prop boats, for example, would make trim tabs a bit more problematic than an interceptor. Similarly, a surface drive would make the intercept a bit more problematic. The hull shape also plays a part in the choice (which can be tailor made to suit one system over another, but with a resulting lesser efficiency at higher speeds)....and depending upon the final 'top' speed which usually has far more importance than the efficiency at 'lower speeds'. Since neither are very effective in this realm.

What you wish for at lower or slow speeds does not hold true for higher speeds, in terms of efficiency. A pressed down tab, would create more drag than an interceptor, but how much...how far is it pressed down, what size tab...who knows??..it depends upon the hull shape and size of tab and size of interceptor etc But if the tab has some appreciable thickness, this provides 'some' buoyancy and may counter some of the drag....who knows.

I think you need to define "better at lower speeds" for each application, as i don't think any conclusion is definitive as a generalisation of one system compared to another.

So as always design is a compromise. In this case which would you like to compromise, the higher speed or lower speed?

[tom28571]

Will,

We know that tabs do work in the lower speed range of 10 to 20. That is the range they are most used for in getting a sluggish boat onto plane or holding a bow down in rough water. I think your question is, "do interceptors work very well in that range?". I don't know either but suspect interceptors do need more flow velocity than tabs to be effective. I did drive a Volvo IPS within interceptors on the transom but have no reference and was so interested in the IPS that I did not notice anyway.

[Yellowjacket]

Each has its advantages, the interceptors act like a "Gurney flap" at the trailing edge of a wing, they increase the lift and moment at the trailing edge of the planing surface at a realtively small increase in drag.

The key advantage of them is that when you don't need them they are retracted and gone.

If you add large trim tabs, you will get more efficient lift at low speeds, and you will also add that lift further aft, which is important to low speed trim, so trim tabs will likely do a better job in the regime that you are looking at. With trim tabs you will have more area, get more lift, the lift will be further aft, and there is a good chance that at low planing speeds you will actually have less drag than either the original hull had or the interceptors.

The downside of trim tabs or an afterplane is that you are adding surface area that you don't need and don't want at higher speeds, so you end up with unnecessary area once you are above the speed where they are helping you.

[Willallison]

This all mirror's my own thought's .... I was just hoping for some experience from others.
In terms of the project I have in mind, it is a 28ft planing hull with a top speed of 32 knots, but is designed to travel at anywhere between 10 & 25. Deadrise is 12 degrees and loaded displacement is up to 3000kg or thereabouts. I'm not looking to stop squatting, rather to ensure that the deeper fwd sections remain in contact with the water to improve ride quality. They may seem like one and the same, but in fact trim never rises above 3.5 degrees. I'd like to be able to reduce that on occaision to 1 - 2. Also to counteract heeling in a cross-wind

[Ad Hoc]

In the application you've noted, tabs are preferable.

[tom28571]

Will,

It was just for that purpose that I developed the aft chines that I use. They act like tabs but are lower drag. Of course they are not adjustable and maintain a lower than usual trim angle which does rob a bit of top end speed. A compromise I am quite willing to make on a cruising boat. I do not experience any appreciable heeling in cross wind. Actually, I've never noticed any at all.

in your case, with a completed hull, I'd agree with Hoc and suggest adjustable trim tabs.

[Willallison]

Tom, I considered adding down-angled chines, but decided against it in order to allow a greater freedom to trim the boat as desired. Top speed is of little consequence to me either, but I like the ability to be able to trim the bow up somewhat under certain circumstances. Also with a top speed in excess of 30 knots, I'd be concerned about the increased risk of dynamic instability as a result of forcing the bow sections to remain immersed.
I'm not surprised that you 'suffer' less from heeling. Mine is a much taller boat with greater windage - a necessary evil of having to include so much accomodation. Also the bow rides well clear of the water when on the plane - the stagnation point is probably 1/3 of the wl aft, so cross winds tend to try to blow the bow 'off'. Steering correction results in some heel....

Cleary tabs add to the wetted surface, though in most cases I've found, adding small tab angles increases rather than reduces the speed... the increased lift more than offsetting the increased drag

[Ad Hoc]

willalison

I wouldn't worry much about this since the skill of the helmsman plays a major part. It all depends how they handle the boat in rough seas...some like to have bow up trim some like to have bow down trim, also how they 'ride' they waves, whether they go direct, head on, or meander up and over to reduce the trim down the slope of the wave etc etc...it all depends upon their own skill and experience.

BUT, having the flexibility of using tabs greatly improves the situation, regardless of the effects of any increase or decrease in resistance.

[Willallison]

Agreed -the greatest variable (and one that the designer has no influence over) is the skill and common sense of the operator.
Having had the rather surprising experience of stuffing a boat into the back of a wave a number of years ago whilst running down-wind, it's not one that I have a great desire to repeat. This was a heavy 26' deep-v boat that required tabs and outboards to be trimmed right down to remain on the plane at 20 knots. The seas weren't overly large - but just large enough for one to end up in my lap....

[Commuter Boats]

I've attached a PDF that hopefully might help. My own thoughts on the subject would suggest that a boat with a high loading per square foot of planning area would benefit from the tabs ( additional planning area) while a boat with a lower loading may benefit from the interceptor plates. I don't know where the line would be.

[Ad Hoc]

Will

Trim tabs are also used for designs that well, ..are not designed properly. The designer does not know the limitations of the hull form and what factors affect the performance (ie from a tank test)...or if they do, but can make no more compromises, the only option is trim tabs to offset the less than perfect design.

Most hulls have tabs because the factors that affect the performance are unknown but...then there is the, wow, look putting on tabs does X or Y...amazing! Where having the tank test data, it would show moving the LCG aft has the same effect, just do this LCG shift in the prelim phase, or change the L/B ratio etc.

I've had a few moments of getting wet from way way too much green seas coming over, on a 7m RIBs in 2m Hs seas, but had full confidence in the helmsman, as i was conducting the trials at that time. We can never account for 'common sense', or lack of...

[tom28571]

I've attached a PDF that hopefully might help. My own thoughts on the subject would suggest that a boat with a high loading per square foot of planning area would benefit from the tabs ( additional planning area) while a boat with a lower loading may benefit from the interceptor plates. I don't know where the line would be.

Had not considered that but it makes sense. Tabs could lower bottom loading on plane while interceptors must increase bottom loading. That is, unless interceptors actually lift more hull surface out of the flow.

I have only stuck the leading wave badly one time but my daughter in law was slightly injured in the process. I neglected to account for the reduced wave period and increased height while approaching shore downwind at speed. Good judgment results from experience and experience is the result of bad judgment. I usually run a bit slower than wave speed in larger ones. Otherwise surfing into the wave ahead is inevitable.

[rbrooks]

We use the QL's on all our 32' boats they work well at speed with less efficiency loss than the tabs. However, at low speeds their performance does suffer. They will work great at 20 knots but lose effectiveness on our boat at 13-14 knots.

[tom28571]

We use the QL's on all our 32' boats they work well at speed with less efficiency loss than the tabs. However, at low speeds their performance does suffer. They will work great at 20 knots but lose effectiveness on our boat at 13-14 knots.

Are you speaking of Fountains?

[rbrooks]

No, These QL's are on the Eastport 32 You can see the boat at www.brooksboatworks.com. However I have had much experience with the Fountains earlier in my carreer

[tom28571]

Very nice Mr Brooks,

It looks like you don't carry the topsides tumblehome well forward of the sheer break as many NC builders do. I prefer the way you do it, if I am seeing the rendering correctly.

[Willallison]

Very nice indeed!
And thanks for your thoughts on the interceptors.... it confirms my expectations.

[HJS]

I would prefer a hull that works efficient in both low and high speeds.
This boat is under construction and we hope to test it soon.
It will have an interceptor over the whole transom.
The test showes that it is possible to create a boat that works efficient in both displacement mode and in planing speed with an aft interceptor.

[Willallison]

Hmm... looks familiar....;) Is it just the angle of the model, or does your chine angle down towards the bow?
HJS, it's not so much a question of efficiency - or indeed of improving efficiency - that I was referring to. In fact the boat in question achieves better than 4 nmpg regardless of speed. It's simply a matter of having the ability to trim the vessel - both laterally and longitudinally to suit the prevailing conditions.
Have you considered the effect of having an interceptor directly in the path of the propwash?

[rbrooks]

Very nice Mr Brooks,

It looks like you don't carry the topsides tumblehome well forward of the sheer break as many NC builders do. I prefer the way you do it, if I am seeing the rendering correctly.

You are correct. Some of the Carolina builders carry the tumblehome too far forward also with an extreme flair. The combination is fairly unattractive to me. I like tumblehome and flair, in reasonable moderation.
We have a new project on the drawing board. We are looking for investors to get it off the ground, if anyone is interested.

[HJS]

Willallison:

Is it just the angle of the model, or does your chine angle down towards the bow?

The chine is in this case paralell to the waterline

Have you considered the effect of having an interceptor directly in the path of the propwash?

It has been done.

HJS

[Willallison]

Interesting looking beast - do you have more pics?

[HJS]

Interesting looking beast - do you have more pics?

The beast!

[yipster]

frend of me put self made longer trim tabs on the bennets, easy done and works well