Prop's performance & trim tabs - need help

peterb · #1 10-12-2008, 12:52 PM

Ok all, here is the lay of the land.

I have a 23' 1972 SeaCraft w/twin Yamaha 200 HP OX66's. The motors are on a full flotation bracket. Also, I have Bennet 12 x 12 trim tabs.

The boat is definately stern heavy.

I have two sets of Yamaha SWS series props (21 x 14.75 and 19 x 15.25)

The engines are mounted at the mid level on the bracket (I have had them at the top level and bottom level as well)

I have two problems.

First, the boat rides (particularly into a head sea) ass heavy. I used to have another 23 SeaCraft w/twin Yamaha 150's (no bracket) and it was a better riding boat into a head sea. That setp had 17" SWS props.

Second, the prop's definately kick up some water when the boat is on plane.

Using the 21" props I get better WOT speed (up to mid 50's) and slightly better fuel economy. The 19" props probably give me a slightly better ride in snotty condition (because they enable me to plane at a slightly slower speed).

Any ideas?

I have been thinking about the following

Purchase and install the drop fin verion of the Bennet trim tabs.

Looking for another set of props (4 blade) that raise the stern a little.

Any specific feedback would be greatly appreciated.

Thanks,

Peter

Bigshrimpin · #2 10-12-2008, 01:03 PM

Peter - Try raising the motors a few holes before switching to different props. I had a similar porblem when my motor was mounted too low. This photo was taken while the boat was coming out of the hole, but you can see the spray. I raised the motor 3 holes and I picked up 400rpm and there's no more spray. Mirage Plus or Rev4's will help. But . . . moving weight forward, bigger tabs, and raising the motors will probably make a bigger difference.

peterb · #3 10-12-2008, 08:29 PM

Thanks for the feedback. I already had movet the engines up to the top mouting point, and I moved it back down one notch because the prop's count come out of the water in rough seas.

Briguy · #4 10-12-2008, 10:37 PM

Peter, there is a guy on the thehulltruth forum called propgods. He could recommend a stern lifting prop. I have twin 115's 4 strokes on my bracket which weigh 416lbs each. I guess they are in the ballpark of your motors. Also can you move batteries forward? I did think your boat rode bow light compared to mine for sure. Mine runs perfectly flat once i got the props dialed in correctly. I run 24" stiletto props. 4 blades may be the ticket.

Fr. Frank · #5 10-13-2008, 07:52 AM

Peter, Tim's right about motor height. You need to go as high as you can without ventilating at neutral trim in medium seas. (You shouldn't be on plane in heavy seas). That said, you've got a lot of weight hanging 2 feet aft of your true planing stern. You REALLY need sternlifting propellers.

Tim's right about which props work well, too. I would only add to consider the PowerTech SCE4, PTC4, & OFS4 props. The OFS4 is very similar to the Mirage+, but a 4 blade, and the PTC4 is similar to the REV4. The SCE4 is like the old Yamaha semi-cleaver, but even better. It's very good for high-speed running offshore in a swell, as it can run partially surfacing before ventilating. The SCE4 does sacrifice some hole-shot performance in favor of it's performance in seas.

peterb · #6 10-13-2008, 09:55 AM

Thanks for the feedback.

I believe that my two motors weigh over 450 pounds each, and when you add the bracket that is a ton of weight.

My two oil tanks are in the enclosed transom and the batteries are under the console.

I really don't care about WOT speed nearly so much as the comfort of the ride (especially since I am going offshore more now).

Brian, I saw that same add about propgods. Also they will let you test put propo I think before buying them.

Thanks again

Peter

gw204 · #7 10-13-2008, 03:44 PM

A set of Mirage Plus wheel pulled the @ss end of my Grady up nicely when on plane. Made it ride like an entirely different boat.

peterb · #8 10-13-2008, 03:46 PM

Spoke to propgods. They are recoommending counter rotating revolution 4's (19" pitch x 14 and 5/8 " diameter)

What you experts think?

Also, does anyone have any leads?

Thanks,

Peter

gofastsandman · #9 10-13-2008, 07:26 PM

Powertechs are probably the best, but pricey.
Bushwacker borrowed my mirage plus and charted real time real world #s vs the his Rev 4. He killed me in hole shot, min plane , time to plane , mpg, midrange , AND TOP END.
Fr Frank, can you explain that?

Peter , I`ve been looking for a 16 p Rev 4 and there were a couple of sets of Rev 4`s on the Hull Truth about a week ago . Think they were 19s and 21s

Fr. Frank · #10 10-13-2008, 10:19 PM

Quote:

Powertechs are probably the best, but pricey.

I agree with that.

Quote:

... He killed me in hole shot, min plane , time to plane , mpg, midrange , AND TOP END.
Fr Frank, can you explain that?

Yes. Slip, rake/rake progression, and percentage of lift/thrust to percentage of diameter.

I went from a Ballistic 14.25"x18"P on my Merc 150, with max rpm of 5650, max speed of 44 mph, and time to plane of nearly 7 seconds, to a Mirage+ 15x17P, with max rpm of 5700, top speed of 47 mph, and time to plane of 5 1/2 seconds, to a Powertech OFS3 15.25"X16.5", with a max rpm of 6050, top speed of 52, and time to plane just under 4 seconds. In all these cases I had a nosecone and cavitation plate mounted 1.5" above hull draft.

A 4 blade prop produces more lift from the inner 50% of the blade surface diameter, and so lifts the stern more. Bow lift comes from the outer portion of the blade diameter. (Yes, you can get both from one prop)

Allow me to use extremes to make a point.
Cleaver props provide up to 75% of their lift and thrust from the inner 40-50% of blade diameter, and work best on stern-heavy boats that rely (nearly) entirely on hydrodynamic lift at speed. They use very little rake progression (change in the forward to rear angle of the blade across it's diameter) to allow for greater ventilation AND less cavitation when fully submerged.

Chopper props (big ears) work best on boats that rely on a good deal aerodynamic lift, but are still significantly water-borne at speed. These props have the highest initial rake, with any significant rake progression occurring only in the outer 25% of the blade diameter, which actually looks like extended cupping of the blade tips.

Recent developments in propellers are blending and melding of these ideas, where you get high rake 4 blade cleavers on extreme speed aerodynamic hulls, and 4 blade low-moderate rake high-cup 4 blades, which are designed to offer high performance without surfacing. The high cup allows them to be run close to the surface without ventilation, and the low rake, fast taper blade provides the needed stern lift.

Most of these recent propeller developments are really beyond my real understanding, as I have been out of the business now for 20 years. It takes a college degree in fluid dynamics and hydraulic engineering to really grasp the new stuff.