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

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pjbrownva

Junior Member
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.
 
Thanks! Anyone have specific props to recommend?

spareparts said:
4 blade prop or larger diameter 3 blade, avoid anything with aggressive rake
Click to expand...

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?
 
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.
 
Update on my prop -- added a doel fin... HUGE improvement!

THEFERMANATOR said:
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.
Click to expand...

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
 
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
 
spareparts said:
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
Click to expand...

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 said:
I ran a zuke DF140 on my v20. Stainless 3x14x20 prop...perfect...no trim tabs
Click to expand...

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.
 
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.
 
Last edited:
SkunkBoat said:
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.
Click to expand...

Thanks for all of the info, @SkunkBoat. I've taken a break to hibernate for part of the winter. :-)

I plan to get back on the water in March, and continue this tuning. I think you're right about needing more pitch and raising the motor height. I *think* I have a 19 pitch Solas 3 blade SS that I ordered and tried for a short while.... so it's basically new. I guess I'll go back to that, raise the motor and report back.

'Til then, wish me sweet dreams! :sleep: (hehe!) Hibernating is fun!
 
spareparts said:
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
Click to expand...

Here's an update.

I ran WOT with a buddy driving, and the anti-cavitation plate was about 3 inches below the surface (!!!)

I bought a used atlas jack plate with a 12" set back.

The jack plate will make it a lot easier for me to fine-tune the engine height.
I know it needs to go higher. I also know that the setback from a jack plate tends to make the motor behave as if it were higher.


I haven't mounted the jack plate yet. Soon (hopefully).

-Philip ***127866;
 
12 inch is a lot of setback, usually only use them for bass boats or when the transom interferes with the engine. You ma have to move some weight around in the boat to help balance it. Normally, when you use setback, it allows you to run the engine higher, rule of thumb, every 4 inches back, 1/2 inch up. So with that setback, you need to mount the engine even higher. Good thing is, having the adjustability will make it easier to get the height right. Downside is, the further back you go, the less stern lift you have(does wonders for bow lift) and it may make it harder to get on plane, especially at lower speeds
 
spareparts said:
12 inch is a lot of setback, usually only use them for bass boats or when the transom interferes with the engine. You ma have to move some weight around in the boat to help balance it. Normally, when you use setback, it allows you to run the engine higher, rule of thumb, every 4 inches back, 1/2 inch up. So with that setback, you need to mount the engine even higher. Good thing is, having the adjustability will make it easier to get the height right. Downside is, the further back you go, the less stern lift you have(does wonders for bow lift) and it may make it harder to get on plane, especially at lower speeds
Click to expand...


Thanks for the info, Spare Parts. I guess it had it all wrong....

So, the good news (really good) is that the 12" setback on this atlas plate is a 6" set back plate with 6" spacers, so I can unbolt the spacers and run it with only 6" setback if necessary.

Still, I think I'll try it with the 12" setback and see.... plus, I use this boat occasionally in a lake in MASS that shallow AND rocky bottom, so being able to raise the motor is a good thing in those cases.

More bow lift isn't a good thing, but well see how she goes!

For weight, I was thinking of moving both batteries to under the passenger seat... but then I thought about it (and looking at the work and additional cost for runs of marine grade cables)... it seems that maybe I should go with some LiFeP04 batteries instead. I've found 2 or 3 brands that now make models designed to put out enough current to use as a starting battery... and each 60 Ah LifeP04 battery weighs only about 15 lbs... so that's a HUGE weight reduction!

Any thoughts on the topic are appreciated...


P.S. -- A bunch of new parts came in this week... I'm not gonna post about them yet. I wanna get them installed w/ some photos, and then I'll post a new thread for each mod / addition.

-Philip
 
take the easy way. Fill a cooler, invite some fat friends, have them move around in the boat, from front to back, move the cooler as well, and see how it changes its running attitude
 
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