Need help with outboard height & prop size

[LoungeLizard]

Guys,

I have a 6m Polycraft with a Suzuki 140 on the back. I need some advice regarding the engine height and changing the prop. At the moment I have:

* OEM 14x19 ally prop
* Engine in the highest hole
* 23kt @ 4500RPM
* 29kt @ 5800RPM WOT
* On the plane the cav plate is still underwater, but it's hard to tell how much.
* Stern sits very low at low speed

What I need is a prop and/or other methods that will lift the stern at all speeds, and improve WOT RPM to 6200 (max for this engine). Not too worried about top end speed but a bit more would be nice. I had a good read of various posts on Ausfish and elsewhere and it looks like a 4-blade stainless might be the go with a pitch of maybe 17 and bit smaller diameter.

Any ideas/input welcomed.

[rowanda]

hi mate, if you need more height you can use a jacking plate or CMC (who make alot of the jacking plates) also make a 5 inch vertical lift plate (I have a pair on my boat) allows you to run a 25 inch shaft on a 20 inch transom. In your case might allow you to run on a low hole and not the highest hole giving you more choice for engine height, also keeps the motor higher out of the water by 5 inches so you can back down on fish or not have to worry about water hitting the cowling of the motor.....as for props, get the height right as you need that as a base to start from, get the info on what prop you are running then and what rpm, speed you are getting and call solas. I would've thought you'd get more than 29knots out if it.

[Swanie1975]

some pics of your set-up would help. who rigged the motor for you?

i started with the 14x19 suzi alloy 3 blade and could spin that out to the rev limiter and was doing about 65kms but my boats only a 5.5 fibreglass weighs around 1150kgs plus POB.

cheers ryan

[LoungeLizard]

Rowanda,

I thought of a jacking plate, but the cav plate is already 80-100mm above the keel of the boat. If the engine is moved much higher the prop would sit behind the boat ie. not have clear water. About calling Solas .... I could do that but I probably won't buy from them (too expensive, yeah I'm a tighwad :). Don't think they'll be too happy helping me when there's no sale in the picture. The Polycrafts are heavy hulls and not known for speed so I'm not expecting a lot more than 30-32kt with 140HP

Ryan,

I bought the boat second-hand so not sure who set it up originally. I lifted the motor 2 holes when I got it which made it much better. It was spraying water everywhere before. I'll take some pics this afternoon.

Any other prop experts here?

[rowanda]

yeah definately pics would help. I wasn't going to go with solas either, but the idea of being able to swap it till i go it right was attractive, saves buying multiple props if it came to that. I could get the same prop from USA for about 1/2 the price or just over but if it's wrong you've lost. My 4 blade stainless was $600 shipped which I thought was pretty darn good

[LoungeLizard]

OK. I've added a few pics showing the engine setup & height. I've also got a 4-blade prop on the way. Further comments, ideas, info welcomed

[Fallen angel]

Have you tried a cavitation plate yet? The next thing that I would be looking at is a stainless prop.

Cheers

[rowanda]

from what i can see from the pics, looks like your cav plate is just inline or above the keel line of the boat...cav plate being the plate about the prop?? or maybe i need new glasses. So could maybe go up slightly...if you have a prop on the way try that and see how you go

[LoungeLizard]

Have you tried a cavitation plate yet? The next thing that I would be looking at is a stainless prop.
Cheers

Do you mean a hydrofoil or Permatrim? No I haven't, was hoping to look at other options before drilling holes


from what i can see from the pics, looks like your cav plate is just inline or above the keel line of the boat...cav plate being the plate about the prop?? or maybe i need new glasses. So could maybe go up slightly...if you have a prop on the way try that and see how you go

It's a bit misleading, the cav plate is actually about 70mm above the heel level, and in the pics the motor was trimmed in a bit more than what I usually cruise at. The motor is already in the highest position. As I've said (maybe you can see that in the pictures as well) the prop is almost starting to get in line with the keel. If I lift it higher the prop will be in the "wash" of the hull and I suspect will ventilate a lot. But I could be wrong .... was hoping some guys here might some insights.

Anyways, the Solas I ordered is supposed to be a stern lifting prop and 4-blade that "bite" better. I'll see how it goes.

[gofishin]

...What I need is a prop and/or other methods that will lift the stern at all speeds, and improve WOT RPM to 6200 (max for this engine)... 1) 4 blade prop, 2) permatim foil, 3) trim tabs. I only put trim tabs last as I'm not sure you can add them to a poly. Look at (2) when you have the 4 bld sorted and are still looking for more lift.


...it looks like a 4-blade stainless might be the go with a pitch of maybe 17 and bit smaller diameter.... I would try a 4 bld between 14 to 14 1/4 diam x 16 or 17 pitch. Smaller diam might be OK too, possibly even better - say 13 7/8 though not sure what is available in the sub 14's. What prop do you have on order?


...but the cav plate is already 80-100mm above the keel of the boat.... Not correct LL, you are looking at the splash plate. To me it looks like the A/V plate (commonly called cav plate) is about in line with the keel, and could even go up. Run a straight edge off the keel and adjust trim so the A/V plate is parallel with the straight edge, then see if it is above or below the keel. Problem is going up means more holes drilled in the transom, so try your 4 bld & hang off the back & have a good look at the A/V plate height. Trim out to see when the prop starts to ventilate, plus see how it behaves in turns & a following sea etc.


from what i can see from the pics, looks like your cav plate is just inline or above the keel line of the boat...cav plate being the plate about the prop?? or maybe i need new glasses. So could maybe go up slightly...if you have a prop on the way try that and see how you go You're on to it rowanda, and your glasses are just fine mate :)
cheers
Brendon

[gofishin]

79214

As you say LL, the motor is trimmed in a little in the pics. The red line needs to be parallel to the keel to get an 'idea' if the motor height is OK. I say idea because it is mostly always confirmed by testing, and sometimes the correct height doesn't follow the theory.

Note where the trim level gauge is when you get it parallel, as a touch out from here will normally be the 'sweet-spot'. Run with the new prop at the s/spot & hang over the back & look at the A/V plate.
cheers

[Goochi]

G'day, from the pics it looks like your motor is a little low but the only way to really know is to get up on the plane and look over the stern and see where your cav plate is sitting. It should not be buried in the water. Ideally it should be in the splash zone. If your boat is heavy the four blade prop should be a good choice and give you better lift. You may not need a jacking plate if you want to raise your motor. You may be able to drill a couple of new holes depending on where your existing ones are.

[LoungeLizard]

Hey Brendon,

Thanks for the detailed reply. I'm talking about the same plate you are. I'll see if I can get a straight edge and measure the gap. I tried that before but is was a bit hard holding and measuring ...

Anyway, moving the engine is not an option without a jacking plate. The top holes through the transom are as high as they can go. If I can get a jacking plate cheap somewhere it might be something worth looking at. Interesting that you would prefer a Permatrim above trim tabs. Why do you think trim tabs wouldn't work? I've read some conflicting things about Permatrims and the idea of drilling holes in the motor scares me :)

The 4-blade I've got is coming 13.75" in ally. If that works well I'll get stainless in similar dimensions

[rowanda]

hey mate, if that prop works then just leave it that!! The plates I have are the vertical lifting plates and from memory were about $300, not too expensive. I'm sure you could have them made somewhere for cheaper, but they are about 13-14mm thick and weigh about 5kg ea...heavy duty thats for sure and I didn't want to stuff around having one made and being drilled out, this one fits up to original holes etc so a matter of just bolting on. I guess it would give you the option of running your motor down on the lowest hole which would probably bring it up a bit, but only if you need to. Fingers crossed the prop is the solution you are looking for

[gofishin]

... Interesting that you would prefer a Permatrim above trim tabs.... Not normally! Only because I'm not sure if they can be fitted to a polly! Otherwise they are by far the best option (over a foil).
cheers

[LoungeLizard]

OK guys, just a quick follow up. I fitted the prop (4x13.75x17), didn't work. Holeshot was better but cruise speed & WOT (had to back off 'cause it went past 6200 RPM) was down and slip was a little worse as well. I'm surprised by the result but there you go. Luckily the prop was cheap so I can use that as a backup or sell.

Anyway, I measured the distance between the underside of the keel and the center line of the ventilation plate when the motor is trimmed parallel with the keel, got 27mm. I was way off with the 80-100m before - well spotted guys! Also noticed that the motor at that position was right at the top of the trim range.

I'm at a bit of a loss as to what to do next. It doesn't look like trim tabs are an option because of the grooves running all the way to the stern. I might have to invest in a jack plate.

[Dicktracey]

http://www.ausfish.com.au/vforum/showthread.php?181496-552X-evo-vs-1900-Bluewater/page4&highlight=552x
just been through exactly same thing with Glass boat mate, few figures on thread may help
dick

[johncar]

It is always hard to say definitively but in theory 27mm with the leg that far behind the end of the planing surface would seem a little low still. I have used the rule of thumb 1" up per foot behind as a starting point but it is a bit hard if you have run out of height without spending money and effort to fit a plate. It is highly possible that you would lift it just to have to lower it again.
I know Solas are a bit more expensive but from my experience their performance guarantee has saved me from having spare props to try and sell later and if you ask for a good deal the difference may not be as much as you first think.
I don't believe that you can also compare the performance of an Ally prop to a SS model of the same proportions either. The blade contours and cupping can be quite different.
But as others have said, the best indicators of optimum engine height and propping is you should be able to trim out to a bit above parallel without ventilation but not unusual to get some ventilation in the last quarter of trim range. If you can trim right out without ventialtion may indicate engine low.
If you are trimmed out and running well and go into a tight turn it is normal to get some ventilation without trimming in but should bring it under control by trimming in below about halfway.
Seeing the Cav Plate just skimming the water surface at speed is another indicator. Anyway I think it is worth persisting, can be the difference between your performance being ordinary to excellent. I haven't had any experience with setting up a Poly boat though so hopefully someone who has will be able to help more.
Good luck with it

[hallcomp01]

If your still worrying about your prop over revving etc then I tend to agree with the lads here and speak to solas Shane or Steve will set you straight, I can't give these guys big enough wraps..
Don't forget the 4 blade ally's are good for bum lift and better bite the solas ones are better made and for 300 odd good buying if your not worried about staying in the red zone all day.. Cheers

[LoungeLizard]

I've been reading up about jacking plates. One thing that worries me a bit is moving the weight back 6" or so. How much extra strain will this put on the transom and will the bum sit even lower? Anybody here use a jack plate before.

Also, I was wondering if I could make a temporary jack plate out of two bits of beefy RHS, say 100x100x5mm. Can anybody see any issues with that?

A big thanks to all that have responded so far.

[Swanie1975]

whereabouts are you located LL? have you got all the figures from running your other props that you have tried?

cheers ryan

[rowanda]

I've been reading up about jacking plates. One thing that worries me a bit is moving the weight back 6" or so. How much extra strain will this put on the transom and will the bum sit even lower? Anybody here use a jack plate before.

Also, I was wondering if I could make a temporary jack plate out of two bits of beefy RHS, say 100x100x5mm. Can anybody see any issues with that?

A big thanks to all that have responded so far.

Hi Mate
I have the vertical extension plates (http://www.cmcsales.com.au/pages/setback.htm part 50012) I'd be worried about using only 5mm plate. I have them on a 130hp outboard and they weigh approx 5kg each and are about 13-14mm thick...awesome and do the job very well...not cheap but predrilled and ready to go, about $300 from memory, I'm sure you'd get some made cheaper at a fabricator but 5mm thick would worry me. Oh and there's no setback to speak of, only the thickness of the plates (13mm)

[LoungeLizard]

whereabouts are you located LL? have you got all the figures from running your other props that you have tried?

cheers ryan

OEM Suzuki 3x14x19 ally Prop

4500RPM @ 23 knots
5800RPM @ 29 knots WOT

Solas Amita 4x13.75x17 ally prop

4500RPM @ 20 knots
6300RPM @ 28 knots

I've sorta given up on the prop idea. I'm sure I can do a bit better with a SS prop but for $500+ I think the gains will not be worth it. Not until I can get the height sorted anyway.

I'm in Gladstone


Hi Mate
I have the vertical extension plates (http://www.cmcsales.com.au/pages/setback.htm part 50012) I'd be worried about using only 5mm plate. I have them on a 130hp outboard and they weigh approx 5kg each and are about 13-14mm thick...awesome and do the job very well...not cheap but predrilled and ready to go, about $300 from memory, I'm sure you'd get some made cheaper at a fabricator but 5mm thick would worry me. Oh and there's no setback to speak of, only the thickness of the plates (13mm)

So, how does that work? Do you need to drill some more holes in the transom? I really don't want to do that unless I'm 100% sure it's going to work. That why I thought of some RHS - using the existing holes one side, drill some new ones higher up on the other side. Could be a bit fiddly to get the bolts done up ...

As for strength keep in mind steel is a lot stronger then ally and RHS will flex a lot less than the angle. It's just temporary anyway, just to see if lifting the outboard does anything. But yeah if i can get my hands on something bigger I might try that, if it doesn't add too many kg's to the transom.