Extending trim tabs

[lumina]

Hi All, Gene from Glittering Image (48'9" 1974 Yachtfish).

I was wondering........(always gets me in trouble !!)

If I were to add say 2 or 3 or 4+ inches to the depth of my trim tabs would that have any positive effect in bringing the bow down and getting on plane ?

Thanks

 

[Walter P]

About 15 or 20 years ago I did just what you are suggesting on a 1980 43 DC. I figured that the tabs that came with the boat from the factory were waaay too short so I called Hatteras and they actually confirmed it. They had very good reason to undersize them however. It seems that although most folks that have 43's with 6-71 ti's assume that they are 390 HP actually few were. Beginning with the 1980 model year boats, they (Hatteras) concluded that with proper engineering and layup procedures they could use a lot less resin/fibreglass and still maintain the proper strength to their boats. This made them lighter and therefore faster than the original "tanks". On some models such as the 43 DC it raised the potential of a very serious problem with broaching while entering a rough inlet or large following seas. Now, we all know that in a following sea a prudent skipper retracts his tabs...right? Well not everyone knows this little tidbit and in order to prevent product liability suits the boat manufacturers reduced the size of the tabs and LOWERED horsepower of the engines. I confirmed this information by speaking to the engineers at Covington Diesel (they provided the marinized Detroits for the 1980 43 with TI's) and they looked up the spec and said that in fact my 6-71 TI's were not 390 like some of the earlier ones but were only 325 HP. This drop was accomplished by resetting the governors lower and installing smaller injectors. Well I wanted more power and I felt that I was a prudent skipper so I had Johnson & Towers come out and reverse the de-tuning while at the same time installing substantially larger tabs. I believe I added about 8 or 10 inches to them. What a ride...... Empty boat except for 2 guys and about 1/4 tank of fuel, we hit slightly over 23K (average of several speed runs in opposite directions). The boat was very squirrelly but since it would not be operated anywhere near those speeds, I left it that way. I believe we had probably the fastest 43 around. When the normal stuff was on including fuel, water etc, I lost only about 1 - 1 1/2 Kn.

One other thing.. If you talk to Bennett they will also tell you that the larger planes will require double cylinders (which I also did) but they would not officially recommend doing any modifications from factory specs. Everyone worries about product liability.

In conclusion....draw your own conclusions

Walt

 

[lumina]

Walt,

Thanks for the quick response and as always the shared knowledge/experience.....

Am I considered a "tank".

My tabs now have a 2" "L" on the end, in other words they are flat and then at the end they are bent up 2 inches, I was thinking of flattening out that "L".

We cruise at 12k (1500rpm) and rarely see 18k, so I don't see it affecting stability.

I'll think it over some more !!

G

 

[Walter P]

Gene,

Well first of all the term "Tank" when referring to a Classic Hatteras is considered complimentery because classic Hatts were built like tanks..very heavy. That is probably one of the main reasons they have held up so well after so many years of service. Lesser quality boats would have suffered much damage given the same use.

If it were my boat (your 48 YF), and I had difficulty getting the bow down and the boat on a good plane, I would definetly extend the tabs for starters. It's a cheap fix and may be all you need especially if your power is marginal. I would not try to straighten out the bend as you describe. That 2 inch flat surface serves a couple of purposes.. one is it makes the plane stiffer, the other is that in a hard reverse, it lessens the possibility of the tab folding under and ripping out the hydrylic cylinders.

My recommendation would be to aquire a couple of 1/8th thick piece of stainless steel plate (316L grade). Cut them to the same width as your existing planes X extend the length as desired. If possible take the planes to a machine shop that has a heavy duty brake to create a similiar two inch 90 deg trailing edge. Drill and countersink a bunch of holes large enough to work with # 10 sae flat head screws. The lock washers and nuts or locknuts should be no problem as they are on the top face out of the slip stream. Make lots of holes to be certain the new plate is secure to the original plane. The leading edge should be beveled just in case the tab was originally installed too low on the transom, necessary to minimize drag.

Oh, one other thing... Drilling and/or machining 316 SS is a PITA and I recommend using either carbide or cobalt drills and countersinks for the job. Have fun and let me know if I failed to explain what I did properly.

Walt

 

[lefteris_gr]

I have a 1989 48 Convertible.

A previous owner extended the depth of the trim tabs by about 2" simply by screwing a bronze block between the tab and the ram - see photo.

Judging by the boat's attitude and behavior, I think this move was in the right direction. In fact, I've often thought of extending them further by installing thicker blocks. Of course, the downside is that when fully retracted, the tabs are not flush with the hull but actually extended by the thickness of the block (2" in this case). In my case, broaching isn't much of an issue, and flaps are fully extended 90% of the time, so this doesn't bother me much.

L.

 

[Walter P]

It seems to me that by using the "block" on your trim tabs you are always running with some down tab, which kind of defeats the purpose of adjustable trim tabs. At some point by adding even more dimension to your blocks you can generate a major performance problem....drag.

I believe you are much better off by extending the depth (fore/aft) because by so doing you are achieving the lift you desire without increasing the drag disproportionatly. Of course at some point it would require additional actuators because of the increased pressure. Also remember that with more actuators it will require additional fluid to be pumped so that if you use the same Bennett pump, it will operate somewhat slower, but it will work.

Walt

 

[Captcoop]

My tabs are boat leveler and I talked with their engineer and he told me that they installed a internal bushing to stop the full extension of the actuator. He said it was at Hatteras request. I replaced all 4 actuators and it was the best money spent short of the repower. If you measure the travel of the tab you can compare it with the mfg. spec.
Good Luck

 

[sgharford]

Very Cool Thread - interesting story Walt about the 43DC. I only recently learned the hard way of the importance of trimming up in following sea, and for that matter, a tall head on sea. The boat handles so much better at the cost of maybe 1.5 knots at the same RPM when tabs down in 3' to 4' seas at 15 knots.

I would think by adding a block, and thus putting more pressure on the actuator then it was intended for in normal operation, you could blow out the actuator or have bolts holding actuator to transom shear.

 

[bigbill]

my boat has fiberglass tabs, what do ya'll think about bolting a longer stainless plate to the existing glass tab, as walter suggested. do you think i would be better off replacing the whole tab with stainless, and having them made longer from the start. thanks, bigbill

 

[TopHattandTails]

A buddy of mine ran a 56MY for 30+ years. He said "the best 2 things you can do to a Hatt MY is extend the tabs and install exhaust elbows to throw exhaust out and into the wake". The tabs are on the list for me, jury's still out on the exhaust (not a problem on the 53). The 56MY owner extended his tabs with SS, but had to add significant bracing so that they would not fold. Kept the same actuators though. Worked like a champ. He said he extended them at least 4"'s. BTW - Lefteres_gr - those are the shiniest props I've ever seen. They get wet after the photo for the first time?

 

[Walter P]

Funny you should mention side exhausts because the same 43 DC also had a problem with exhaust smell in certain conditions. Depending on our speed or wind direction it became unbearable driving with the aft deck windows open. We decided that by fabricating elbows out of epoxy/fibreglass and routing them out the sides about a foot forward of the stern (I don't remember the exact distance, but I can get it if anyone wants it). By molding a raised lip on the leading edge of the oval opening on each side it directed the water flow a little to aid the flow of exhaust out and away from the hull. By the time the stern vacuum would get hold of it we were long gone. Well, we did it and it worked extremely well. Never had the exhaust smell again and the carbon monoxide detector on the aft deck stopped sounding off. The distance forward is important for several reasons, one is effectiveness and the other is the limited space because of 2 mufflers on each side. I can get some pictures of the job if anyone is interested.

Walt

 

[Canuck Dennis]

Big issue I have on my 43 dc is exhaust noise, boat has 310 HP J&t 6-71's but I think the exhaust pipes through the boat are designed for the smaller HP, so we get the loud quite drone,, everyone hears us coming from a long way off boat does have Veratones but just not enough pipe size I think

 

[lefteris_gr]

It seems to me that by using the "block" on your trim tabs you are always running with some down tab, which kind of defeats the purpose of adjustable trim tabs. At some point by adding even more dimension to your blocks you can generate a major performance problem....drag.

I believe you are much better off by extending the depth (fore/aft) because by so doing you are achieving the lift you desire without increasing the drag disproportionatly.
Walt

Sure it's always running with some down tab, but it's still as adjustable as before - it's got exactly the same range of movement in terms of distance traveled and it doesn't defeat the purpose.

On this particular boat, in practically all sea conditions the cruising speed is best (for the same RPM) when the tabs are fully extended. I sense speed would be higher if the blocks were thicker. Keep in mind I have a massive swim platform (800-900lbs) plus davits, rib and passerelle hanging on the transom.

You might be right that with this method drag is increased disproportionately as compared to lengthening the tab blade area. I'm not convinced, but it's a rather complex subject and difficult to prove / disprove. Have you seen the "Humphree Interceptor" trim blades used in many new boats?

My point is that inserting an extension block is a 10 minute job with zero cost which some might considering worth trying before (or instead of) replacing the tabs with longer ones.

L.

 

[Walter P]

You're correct that it's relatively cost free (haul out and labor-yours) so in that situation you have very little to lose. It sounds like you have a bunch of stern weight on your boat so I agree that you need to do something. Please let us all know how the additional block extensions work...I have doubts because of the drag.

By the way, adding longer planes is not a terribly big job either. If you don't improve performance and your engines are ok (no extra fuel burn or overheating because of drag), then you may want to consider longer planes.

Dennis.... I believe you have 6" pipes, whereas the TI boats have 8". I don't know why but there has been much discussion about that on this forum in the past and apparently all the TI boats make less noise than the N's. Actually your 310 hp J & T's are juiced up N's. The standard N's only ran something like 265 -285 hp. In any event the cost of installing 8" exhausts on your boat would most likely cost much more than you think. One of our forum members had his 6" system replaced completely with 8" and it cost him right around 10 K. I don't think it made any difference in the sound of his exhaust but the engines probably breath better.

Keep in mind that most everything I'm stating is based on opinion and we all know that story.

Walt