Prop advice... need more stern lift... Add hydrofoil?

pjbrownva · #1 10-12-2020, 10:32 AM

I have a 1984 V20 cuddy with the 60 gal in floor tank.

My motor is a 2003 Johnson (Suzuki) 140 HP 4-stroke.

I added Bennet hydraulic trim tabs, and they are great....

However, at speed, I usually have to have them about half-way down to keep the bow down and avoid pounding.

Since the trim tabs provide noticeable drag, I realized that I need more stern lift... either via a different prop, and/or maybe adding a doel fin.

My first preference is a different prop. Right now I have a Suzuki 17 pitch x 14" 3 blade stainless steel prop... it's made for that motor.... RPM-wise, I know I can go down to even a 15 pitch... but I'm thinking that a different prop design would be better... one that provides more stern lift than factory prop.

In my mind, I would like to be able to cruise without using the trim tabs, and have the bow stay down, or only have to barely use them... that way I've got less drag.

Thoughts? Advice? Experience?

P.S. -- I like the doel fin... I've had them on fresh water boats... I just don't know if they are bad in salt water... For example, are they bad in a following sea? I'm wondering if (just like trim tabs) the waves and sea could push down on the doel-fin, and increase the risk of taking a wave over the stern.

spareparts · #2 10-12-2020, 05:59 PM

4 blade prop or larger diameter 3 blade, avoid anything with aggressive rake

pjbrownva · #3 10-12-2020, 07:38 PM

Quote:

Originally Posted by spareparts

4 blade prop or larger diameter 3 blade, avoid anything with aggressive rake

Thank you, @spareparts, For my motor it's spec'd for up to 14" diameter, so I guess I'm looking for a four blade prop with a 15 pitch, and not a lot of rake.

The factory Suzuki prop has aggressive rake...

I was looking at the Mercury Inertia (With the swapable hub system)... but I only saw that prop in 15"+ diameters... at least where I looked.

Anyone have a good prop to recommend for this use?

THEFERMANATOR · #4 10-12-2020, 08:05 PM

Definitely look into a 4 blade prop. Just keep in mind 4 blade props tend to have less prop slip, so you may need to step down in pitch. It's always best if you can find a prop shop willing to work with you so you can try out different props.

pjbrownva · #5 01-04-2021, 06:28 PM

Quote:

Originally Posted by THEFERMANATOR

Definitely look into a 4 blade prop. Just keep in mind 4 blade props tend to have less prop slip, so you may need to step down in pitch. It's always best if you can find a prop shop willing to work with you so you can try out different props.

Thank you @thefermanator, here's the latest updated...

I added doel fin to the outboard, and WOW! I now top out at 35 MPH, and can spin the prop up to 6200 easily. That's a HUGE difference, and now the bow stays down a lot more even without using the trim tabs!

In other words, I think that this suzuki 14in x 17 pitch stainless steel prop is a great match for my boat.

My next question is about engine height. The prop shaft is currently mounted in the middle hole, which puts the shaft approx 8 inches below the bottom of the boat.

Any feedback on what height is ideal? I'm thinking that the motor should be higher, but don't exactly know, since my prior experience is with 20" motors on fresh water lake boats.

I'll also try searching the forum for any previous posts on this topic.

Thanks!

-Philip

spareparts · #6 01-05-2021, 08:13 PM

generally speaking, the higher up you can run the engine, the more efficient it will perform, but with everything, there will be trade offs. Easy way to check it, have someone run the boat in a normal fast cruise and trim position, look over the stern and see if you can see the anti-ventilation plate on the engine(the thing you bolted the fin to). It should be just out of the water at a good fast cruise and proper trim position. The fin should be skipping across the top of the water. Be careful, don't fall in. If you have time to play with it, raise the engine up till it blows out in turns then go one hole back down. The higher you raise it, the less leverage you will have

pjbrownva · #7 01-06-2021, 07:17 PM

Quote:

Originally Posted by spareparts

generally speaking, the higher up you can run the engine, the more efficient it will perform, but with everything, there will be trade offs. Easy way to check it, have someone run the boat in a normal fast cruise and trim position, look over the stern and see if you can see the anti-ventilation plate on the engine(the thing you bolted the fin to). It should be just out of the water at a good fast cruise and proper trim position. The fin should be skipping across the top of the water. Be careful, don't fall in. If you have time to play with it, raise the engine up till it blows out in turns then go one hole back down. The higher you raise it, the less leverage you will have

Thank you. I was considering putting a jacking plate on the boat, but was then advised that they aren't recommended on this type of salt water boat.

I don't know if the additional setback from a jacking plate would be a help or a hindrance.

I did note that Suzuki took a victory lap for designing the outboard to move the center of mass of the motor further towards the bow... which definitely had me wondering if a jacking plate is a good idea or a bad one.

Opinions are welcomed, and facts are GREATLY appreciated.

SkunkBoat · #8 01-06-2021, 07:28 PM

I ran a zuke DF140 on my v20. Stainless 3x14x20 prop...perfect...no trim tabs

pjbrownva · #9 01-06-2021, 08:42 PM

Quote:

Originally Posted by SkunkBoat

I ran a zuke DF140 on my v20. Stainless 3x14x20 prop...perfect...no trim tabs

Wow! Do you remember how many RPM's you could turn with that prop?

I'm thinking that if I raise the engine height some, I'm likely to get more speed and higher RPMs, which might even allow me to go from my current 17 pitch prop up to a 19 or 20 pitch prop... assuming that I could still reach the recommend max RPMs at full throttle. I've read that these 'zuki 4 stroke engines like to be able to rev, and don't do too well when they are over-propped.

SkunkBoat · #10 01-07-2021, 08:07 AM

This is from an old post

"It's a Suzuki 3 blade stainless 14 x 20. It has vent holes at the base of each blade. It's the original prop that the dealer fitted when he installed the motor. I run fairly heavy, with lots of rods, gear, anchors, coolers, and always 30 -55 gallons of gas. The hard top adds some weight too. At WOT, light, with motor trimmed up perfectly, she will get to 6000rpm and ~38mph. But I rarely do that. When I'm in a hurry I'll run at 5600 rmp, just shy of WOT.

Do a search on the Tag "DF140" for more info.

zukes have a 2.5:1 gear ratio. They need a big prop. I suspect you have the 17" prop trimmed in too deep to keep it from overrevving. 17 is too small.

You should be able to run fine with no tabs and definitely without a fin.

And the manufacturers have a range for WOT for a reason. Don't stress about getting to the max of the range. Get the boat to perform well out of the hole and cruising.
Testing should be done without tabs and fins and crap.