Need full tabs to plane, go 4 blade?

[Finnerty]

My boat is a 26 Tollycraft flybridge cruiser
10000 lbs, single 454 V-drive 1.99:1 reduction
This is a 1977 Mercruiser WOT spec range 3800-4200 rpm
I've got room for a max 19" diameter prop
Original 19X20 3-blade gave WOT 3700 (too low)
I repitched:
Now 19X18 3-blade gives WOT 3850 (better)
It's a stern-heavy boat that needs full tabs to cruise at 3200 rpm giving 16-18 knots
I'm happy with the performance, lots of torque, stays on plane but wondering if a different prop eg 4-blade would improve fuel economy at cruise speed by lifting the stern and reduce reliance on the trim tabs?
Also would like to get to 4000 rpm WOT for (I assume) better engine longevity.
So if a 2" pitch reduction gave me a 150 rpm increase with my 1.99:1 reduction then a further 2" to 19X16 should give me another 150 rpm WOT?
What 4-blade size should I consider?

ie>
19X20 3-blade = 3700 rpm WOT known
19X18 3-blade = 3850 rpm WOT known
19X16 3-blade = 4000 rpm WOT I guess?
??X?? 4-blade = 4000 rpm WOT ??????

 

[rallyart]

Dropping pitch more will be the best answer. A four blade might help but if you are cruising and need full tabs the issue is more in how boat is weighted. The V-drive is already lifting the hull some so you have a real issue there. At 16" pitch a four blade should be useful getting on plane but not necessarily for economy while cruising.

 

[Scott Danforth]

its a boat, if you want economy, get a boat with oars or sails.

in my opinion, your still running the boat with too much prop and lugging the motor.

 

[steelespike]

A 19 X 16 will get you up on the water easier and stay on plane easier,with less fuss from changing conditions.
wot will be slower .
If you want to get a real indication of your mpg you need a flow meter.Some give real time mpg. Best mpg could be
barely on plane and up to 1000 rpm more.Of course depending on your planing rpm.
I don't think full tabs is the best cruising speed. With the flow meter you can easily play with tabs,rpm,and props and see the results
in a mater of minutes. The difference between a good setup and a perfect set up would require a flow meter to tell any way.

 

[Finnerty]

A 19 X 16 will get you up on the water easier and stay on plane easier,with less fuss from changing conditions.
wot will be slower .
If you want to get a real indication of your mpg you need a flow meter.Some give real time mpg. Best mpg could be
barely on plane and up to 1000 rpm more.Of course depending on your planing rpm.
I don't think full tabs is the best cruising speed. With the flow meter you can easily play with tabs,rpm,and props and see the results
in a mater of minutes. The difference between a good setup and a perfect set up would require a flow meter to tell any way.

Yup, I've got a flo-scan: here's the numbers,

2800 rpm 14 knots 1.33 nmpg just planing with full tabs
3200 rpm 17 knots 1.24 nmpg full tabs
3400 rpm 18 knots 1.12 nmpg full tabs
3850 rpm 23 knots 1.10 nmpg half tabs (WOT)

If I back off on the full tabs at <3400 rpm I scrub speed.
Between 3400 and 3850 WOT the four barrels are open, speed and fuel consumption jumps, but nmpg stays pretty much the same.
I have to adjust the trim over 3400rpm to get these numbers.

I tried a couple of prop calculators and they give a range for a 3 blade prop of 20X14 to 20X20 and a similar wide range of pitch for a 4 blade prop but with a 19 dia.
Not really helpful.

So maybe to refine my question:
19X16 3 blade 4000 rpm
19X?? 4 blade 4000 rpm

 

[steelespike]

My boat is a 26 Tollycraft flybridge cruiser
10000 lbs, single 454 V-drive 1.99:1 reduction
This is a 1977 Mercruiser WOT spec range 3800-4200 rpm
I've got room for a max 19" diameter prop
Original 19X20 3-blade gave WOT 3700 (too low)
I repitched:
Now 19X18 3-blade gives WOT 3850 (better)
It's a stern-heavy boat that needs full tabs to cruise at 3200 rpm giving 16-18 knots
I'm happy with the performance, lots of torque, stays on plane but wondering if a different prop eg 4-blade would improve fuel economy at cruise speed by lifting the stern and reduce reliance on the trim tabs?
Also would like to get to 4000 rpm WOT for (I assume) better engine longevity.
So if a 2" pitch reduction gave me a 150 rpm increase with my 1.99:1 reduction then a further 2" to 19X16 should give me another 150 rpm WOT?
What 4-blade size should I consider?

ie>
19X20 3-blade = 3700 rpm WOT known
19X18 3-blade = 3850 rpm WOT known
19X16 3-blade = 4000 rpm WOT I guess?
??X?? 4-blade = 4000 rpm WOT ??????

I doubt that rpm is lineal. It appears that 4 blade would need to be a 15" pitch to maintain the rpm realized with a 19 X 16 3 blade.
Perhaps the 4 blade would help to keep the stern up at cruising speed.?? Being a 4 blade the diameter may be reduced a little.

 

[alldodge]

I think your biggest problem is its a V-drive. These drives are great for less maintenance but flat out lousy for performance, some much is lost when the hull starts coming out of the water

 

[Finnerty]

I think your biggest problem is its a V-drive. These drives are great for less maintenance but flat out lousy for performance, some much is lost when the hull starts coming out of the water

Agreed, the other issue is the extra weight I carry in the boat, all of it aft: tender, kicker, larger water tank etc.
That's not gonna change.
So if there is no magic prop to get my fat stern up at cruise rpms I am now focussed on optimizing my prop so that I am not lugging the engine at cruise rpm.

I calculated my Apparent Slip with my current 19X18 prop:
2800 rpm 14 knots 33% just on plane
3200 rpm 17 knots 29% cruise
3400 rpm 18 knots 29% cruise
3850 WOT 23 knots 20%

If I re-prop with a 2" less pitch 19X16 and ASSUMING NO CHANGE in slip, my calculated speed will be:
2800 rpm 12.4 knots 33% not quite on plane
3200 rpm 15.1 knots 29% cruise
3400 rpm 16.0 knots 29% cruise
4000 WOT 21.2 knots 20%

I'm assuming this is a worst case scenario as my slip with a lower pitch prop should be lower.
Am I right?
What about cavitation, wi​ll I be opening myself up to that with a lower pitch?

So my conclusion is that reducing pitch 2" in order to increase WOT from 3850 to 4000 to avoid lugging at 3200-3400 cruise will reduce my cruise speed by up to a maximum of 2 knots.
I wonder if I am really lugging the engine with my current WOT of 3850 (spec is 3800-4200) as Scott suggests? If not then no change is warranted (especially considering more than half of my running time is at trawler speed).

I appreciate the feedback AllDodge, steelespike, Scott and rallyart. I'm learning with your help as I work through this.
Excuse me for geeking out with the calculations, but boating season is over and need something to do

 

[alldodge]

In most cases the slip decreases as the pitch is reduced. That said, all boats are different in how they react. If you can find someone to let you borrow and try that would be the way to go. Have you had a close look at your hull and prop? Looking for growth and any damage to the blades

Don't think cavitation should be an issue with a different pitch if it is not happening now

 

[steelespike]

With the 16" it may take less throttle to make your rpm settings.Also it may stay on plane with less fiddling
The boat may improve at other settings as well.
Don't know if a 10000 lb boat can feel peppier but it might.

 

[Scott Danforth]

could always dig into the motor and get another 100+ HP with a few mods (cam, induction, etc.) after 40 years what is the compression like on the motor. it may warrant a 496 stroker kit

 

[Finnerty]

In most cases the slip decreases as the pitch is reduced. That said, all boats are different in how they react. If you can find someone to let you borrow and try that would be the way to go. Have you had a close look at your hull and prop? Looking for growth and any damage to the blades

Don't think cavitation should be an issue with a different pitch if it is not happening now

Bottom is clean, prop is good.​
Another 100 hp would be nice, but this engine was rebuilt 500h ago and being 38 years old, I doubt I will invest more money in it.
Thanks for your help.
Cheers