Speed increase from trim tabs?

[500dollar744ti]

I have a 23' Cobia cuddy w/ 270hp Mercruiser that has always been sluggish to get out of the hole no matter the prop. Some props are better than others but it just doesn't jump up as fast as I'd like.

I bought a set of Bennett Sport tabs with dual actuators on each side for $200 used, I haven't installed them but I expect a dramatic increase in performance overall once they're on.

My boat has a relatively deep vee for its size and doesn't get the top speed other boats of the same size/power, but does handle very nice in rough seas and the ocean.

Part of my issue is that the boat really digs in at the stern when getting on plane, if enough time passes before planing out I'm getting a fuel cut either from the anti-siphon valve or the extreme carb angle. Either way, as long as I can get on plane in a reasonable time, I don't have the issue. It only happens when I have a lot of weight in the back and the boat continues to plow instead of getting up. If everyone moves up front, it planes right out.

I don't like having to ask people to get up front so I can plane out and I don't want to sacrifice lots of top end for a reasonable hole shot. It sounds like I'm asking for too much but my thought is that the trim tabs are my ticket to having what I want.

I'm thinking that with the tabs it will ride higher on plane creating less resistance for top end and with lots of weight in the back, having the tabs deployed will allow me to still jump up on plane easily.

Question is, how much better can expect my holeshot to be with the tabs and what, if any top speed increase can I expect?

 

[tpenfield]

Re: Speed increase from trim tabs?

The lift in the stern will come at the sacrifice of speed. So, your speed may be a bit less, depending on how much trim tab you use, etc.

 

[500dollar744ti]

Re: Speed increase from trim tabs?

If I have the tabs fully retracted on plane shouldn't it perform as it does now for top speed or will I still sacrifice top speed by having them there.

 

[Slip Away]

Re: Speed increase from trim tabs?

Tabs deployed = Increased holeshot and lessened time to plane.
Tabs retracted fully = Same top speed as without tabs in MOST conditions

 

[dingbat]

Re: Speed increase from trim tabs?

If I have the tabs fully retracted on plane shouldn't it perform as it does now for top speed or will I still sacrifice top speed by having them there.

What does an airplane do to decrease air speed? If lowers it's flaps. The same goes for trim tabs.

The benefits of tabs come at a cost elsewhere. I have a fuel flow meter on my boat. The lower you put the tabs, the more fuel you use. Reverse the tabs and fuel consumption goes down.

Tabs are not miracle workers. Sounds like your under powered or over loaded. The increase to planing time is mere seconds in most cases. Chances are you'll get a bigger bang for your buck switching props.

My boat has a relatively deep vee for its size and doesn't get the top speed other boats of the same size/power

What is your top end? What are you comparing it too? The same boat with 200hp outboard will run circles around a 270hp I/O because of the weight advantage

 

[H20Rat]

Re: Speed increase from trim tabs?

Tabs do not ALWAYS lower speed. My jet boat is actually quicker with it slightly down. There are two opposing forces that are working, a) how much hull is in the water, and b) how perpendicular your thrust is to the waterline. (any vertical component to thrust is wasted thrust, it isn't moving you forward) In the case of lots of jet boats, 'b' ends up being more important than 'a', and even though I'm taking an efficiency hit from more wet hull, the increase in thrust makes up for it.

(at the top end I see about 1 to 1.5 mph increase, at cruising speed around 35-40, i'll see up to about 3mph by dropping the tab a little with no adjustment to throttle.)

 

[captain zac]

Re: Speed increase from trim tabs?

flaps on an aircraft are not used to decrease speed
when lowered they increase the camber of the wing increasing lift
allowing the aircraft to fly at slower speeds before stalling
lowering the stall speed makes for a safer landing and a landing using less runway

lowering the flaps will increase drag some on an aircraft if you want to increase drag you use speed brakes
these increase drag on the top and the bottom of the wing


I some cases trim tabs on a boat can increase speed
a boat will lose speed if it is allowed to bounce up and down on plane
so if you use the tabs to prevent the bouncing then the hall of the boat will be more efficient

Harry

 

[spdracr39]

Re: Speed increase from trim tabs?

I did not lose any top end when I installed my smart tabs. It did require moving between adjustment holes to dial them in and making sure I had the correct prop for the job. I bought my tabs first and then picked a prop to complement them. If it is a big deal get hydraulic or electric tabs so you can adjust them as needed and you will have the best of both worlds.

 

[haulnazz15]

Re: Speed increase from trim tabs?

What does an airplane do to decrease air speed? If lowers it's flaps. The same goes for trim tabs.

The use of flaps in aircraft is not for decreasing speed. Lowering flaps allows you to increase the decent angle (rate) without increasing speed. Speed increase in aircraft is solely due to thrust from the engine and the pitch of the aircraft. The slowing down of an aircraft when using flaps has a bit to do with drag, but is mostly due to them being used in the decent/landing phases of flight where approach airspeeds are desired.

Now if you said that spoilers or air brakes being deployed on aircraft are for shedding speed, then you would be correct.

 

[agallant80]

Re: Speed increase from trim tabs?

I think what tabs do depend on the hull design. You won't get more speed out of them, they may help you get on plain faster. My boat the only thing the tabs do is correct leaning from side to side.

 

[500dollar744ti]

Re: Speed increase from trim tabs?

What is your top end? What are you comparing it too? The same boat with 200hp outboard will run circles around a 270hp I/O because of the weight advantage

Maybe I should retract the part about not being as fast as other boats, I've been doing some comparing and it's pretty fast I guess. I'm getting 49mph@4400rpm WOT (GPS) with my current prop (14.75x19P SS) which is sluggish to plane with a full load. If I re-install the Doel-fin that came with the boat, I get 45mph WOT and better plane out though still not great.

What I'm trying to do is lose the Doel-fin and add stern lift through trim tabs. If I keep my current prop, I'm happy with top speed. Out of the other props I've tried, a lower pitch (15x17p) helps me plane out but it over-revs and I can't get over 38mph. So then I tried a 14.25x18p four blade stainless prop, its hole shot was slightly better but not great, then 42mph WOT, not worth the improvement over the 19p I am running now.

The best I ever saw with this boat was 51mph@4400rpm using a 20p stainless prop and a nose cone. The lower unit with the nose cone failed (1.5 ratio) and I got an SEI with 1.47 ratio now. I had to go back down to a 19p because the 20p was lugging to get to the minimum WOT RPM. I've also tried a Turbo 14.25x19 with the vent holes and it didn't do any better, hole shot the same as my current prop with lower top end.

So now, I'm happy with the 14.75x19p HR-titan SS 3-blade I bought from iboats. My top end is good and ride is good, hole shot still lacking as it has been with most props I tried.

My whole purpose in getting the tabs is I want to get the boat on plane faster, ride better in rough seas and have the stability control from side-to-side. I was just wondering if it's possible to see 1 or 2 mph top end with the added lift. Seems like the consensus is that top end will be the same, which is fine but I'm curious as to what others have seen on hole shot with tabs on an I/O boat.

 

[haulnazz15]

Re: Speed increase from trim tabs?

Well, if the tabs don't take care of the issue entirely, you might try a 4-blade prop. They add a touch of stern lift and get more bite out of the hole.

 

[rallyart]

Re: Speed increase from trim tabs?

Yes, put the tabs on. They will help you get out of the hole much faster. They also allow you to adjust the list of the boat as you run so the ride is more comfortable, without having to get people to move from one side to the other. The third benefit is that you can adjust the attitude of the hull in a storm to keep the bow up, or down, with more precision than just using the engine trim. This can make your boat safer and also allow you to run at a slower speed with good visibility and control if needed.

You fuel use will go down because you will spend much less time plowing water at full throttle getting on plane, and more time at part throttle cruising. Helm adjustable tabs are a great addition to most boats.

 

[crabby captain john]

Re: Speed increase from trim tabs?

The LARGE Sport Tabs are designed for 20 - 23' boats. You are at max with a 23'er. These should work fine.... I always go just a little bigger

 

[rbh]

Re: Speed increase from trim tabs?

I would try adjusting the preload a little lighter, what size prop? the neighbours boat is almost the same size as yours and he is running a 15x19, 4 blade, and when empty is hitting high 40s low 50s mph.

 

[bruceb58]

Re: Speed increase from trim tabs?

My whole purpose in getting the tabs is I want to get the boat on plane faster, ride better in rough seas and have the stability control from side-to-side. I was just wondering if it's possible to see 1 or 2 mph top end with the added lift.

Just getting rid of the Doel Fin is going to increase your top end. Adding the Bennet Tabs is a no brainer.

 

[500dollar744ti]

Re: Speed increase from trim tabs?

The LARGE Sport Tabs are designed for 20 - 23' boats. You are at max with a 23'er. These should work fine.... I always go just a little bigger

I say sport tabs but they are an older generation, came from a mid-90's go-fast boat, 26' or 28' maybe? They look just like the sport tabs having dual hydraulic actuators but I'm not sure of the actual model. The indicator looks like this so maybe someone else knows the model..

 

[Tabman]

Re: Speed increase from trim tabs?

That's an older indicator that we have not made in about 5-7 years. Can you tell me the dimensions of the Trim Tabs?

 

[500dollar744ti]

Re: Speed increase from trim tabs?

I believe it's 12x12 but I'll have to measure it.

 

[JoLin]

Re: Speed increase from trim tabs?

Well, if the tabs don't take care of the issue entirely, you might try a 4-blade prop. They add a touch of stern lift and get more bite out of the hole.

Ditto that. My very stern heavy 21' Pro-Line responded nicely to both. Mount the tabs and consider a 4-blade if you want a little 'extra help'