Help me minimize ventilation

[whosmatt]

Hi all....

My answers to the questions:

1. I'm getting more ventilation that I think I should in turns and in following seas. The latter is particularly scary as it was bad enough over the weekend to make me think I had a spun hub. I simply didn't have the power there to control the boat like I wanted.

2. Current prop is the stock 11.1x13 3 blade aluminum that came with my new Tohatsu 50hp. Their part number is 3T5B645270.

3. 11.1x13

4. WOT, trimmed out, is about 5600 rpm with a light load. Speed is about 33-34mph.

5. Tohatsu TLDI 50HP MD50B2EPTOL, purchased new in 02/2012.

6. Boat is 17' CC skiff, home built. I'm unsure of the weight, but expect it to be in the 800lb range dry.


The motor is mounted with the plate level with the bottom of the hull.

Looking for advice on propping for better navigating in all sea conditions.

Thanks,

Matt

 

[BaileysBoat]

Re: Help me minimize ventilation

Your numbers are good. How does the rig behave if you trim down for turns? Is the skiff in trim? Maybe all you need to to is transfer more weight to the stern.

 

[whosmatt]

Re: Help me minimize ventilation

Your numbers are good. How does the rig behave if you trim down for turns? Is the skiff in trim? Maybe all you need to to is transfer more weight to the stern.

It's better when trimmed down obviously, but it still ventilates more often than I'd like when executing turns at speed (and by "speed" I mean a comfortable cruise, not WOT). Trim is level when sitting at the dock; the scuppers are just above the waterline like I'd expect.

Battery is in the console and I don't really have a place to put it in the motorwell without it being below the waterline, which is a no-no for me. I should elaborate on the following seas bit... it really only caused me trouble once, and I may have been in a serious current or had some kelp fouling the prop at the time. Conditions weren't such that I could check just then, as I needed to be able to maintain headway. I was able to maintain about 5 knots at 1500 or so rpm but any more and the motor would really rev up... this was trimmed all the way down. Once I made it into the harbor entrance, it was fine, and I stopped and checked for fouling but found none (but it could have cleared itself in the meantime).

Most of the time it's fine, just looking for that little extra something, as the motor is already as low as it will go on the transom.

Also, the boat has a skeg that ends 18" before the transom. I've heard that in a tight turn, the skeg will bite and allow air to be sucked in behind it. Here's a link to the boat design if this helps: http://bateau.com/studyplans/FS17_study.htm?prod=FS17.

I need a backup prop anyway, so I'm looking for something that will give me a little better performance (not speed, just handling) in the kind of boating I do without breaking the bank. My current prop will become the backup in this scenario. I ran a Piranha on my last boat and loved it; it performed great and was tough as nails, but I'm thinking of trying a four blade and they don't have one for my motor.

Thanks for the help; this board rocks.
-M

 

[Philster]

Re: Help me minimize ventilation

You need to lower your engine, if possible. I think that test is warranted.

The rule of thumb (AV plate even with bottom) is just a guide and is great for a boat that is V back to the stern. If you can lower the engine, please give that a test.


EDIT: nevermind. I just re-read your last post. :facepalm:

 

[steelespike]

Re: Help me minimize ventilation

That skeg probably helps the boat to track better but could cause problems at speed. In the old days they suggested
removing up to 18" of a normal keel when installing an outboard.
We'll forget that for now.I see a 13" 4 blade and an 11" 4 blade here at boats .
when going to a 4 blade it is usually suggested to drop one inch in pitch to maintain rpm.
The 11" would probably put you at about 5700.There must be a 12" 4 blade out there somewhere.
In desperation you could have a prop shop add some cup or pitch to lower the rpm if a 11" 4 blade tests high.

 

[whosmatt]

Re: Help me minimize ventilation

That skeg probably helps the boat to track better but could causer problems aT speed. In the old days they suggested
removing up to 18" of a normal keel when installing an outboard.
We'll forget that for now.I see a 13" 4 blade and an 11" 4 blare here at boats .
when going to a 4 blade it is usually suggested to drop one inch in pitch to maintain rpm.
The 11" would probably put you at about 5700.There must be a 12" 4 blade out there somewhere.
In desperation you could have a prop shop add some cup or pitch to lower the rpm if a 11" 4 blade tests high.

I've got a little room for my WOT rpm to increase (or decrease, really); i'm looking for 5150-5850. The boat is new, so it's never going to get any lighter.

 

[whosmatt]

Re: Help me minimize ventilation

That skeg probably helps the boat to track better but could cause problems at speed. In the old days they suggested
removing up to 18" of a normal keel when installing an outboard.
We'll forget that for now.I see a 13" 4 blade and an 11" 4 blade here at boats .
when going to a 4 blade it is usually suggested to drop one inch in pitch to maintain rpm.
The 11" would probably put you at about 5700.There must be a 12" 4 blade out there somewhere.
In desperation you could have a prop shop add some cup or pitch to lower the rpm if a 11" 4 blade tests high.

Would the reduced diameter of a 10.5x13 4 blade help make up for the rpm loss from the extra blade? my current 3 blade is 11.1x13

 

[steelespike]

Re: Help me minimize ventilation

Most of the time when you go to a 4 blade you will lose rpm and speed
Thus the need to drop an inch to counter some of the loss.Ive seen speed loss up to 3 to 5 mph.
Even with the drop in pitch.
It should help stay on plane better at reduced throttle.
Most 4 blades have reduced diameter.But its still a 13 so you may lose 100 to 200 rpm
While the 4 blade should combat venting and improve hole shot the same pitch
may dampen the improvement, and increase speed loss..

 

[whosmatt]

Re: Help me minimize ventilation

Aha. Solas's site shows an Amita 4 in 10.5x12 that'll work with my motor. I'll try and track one of these down and give it a shot.

edit: it's actually 10.8x12, part number 5313-108-12

edit # 2. If i search by part number, it does show up on iboats, but out of stock.

 

[whosmatt]

Re: Help me minimize ventilation

You need to lower your engine, if possible. I think that test is warranted.

The rule of thumb (AV plate even with bottom) is just a guide and is great for a boat that is V back to the stern. If you can lower the engine, please give that a test.


EDIT: nevermind. I just re-read your last post. :facepalm:

I looked at the setup when I got home and there's about 1/4" of space between the bottom of the bracket and the top of the transom. The motor was mounted by my dealer. I *could* remove the motor, fill the current holes with epoxy and wood flour, and re-drill but is 1/4" really worth it? I'm sure they just hung their template bracket on the transom and drilled without thinking about it too much.


EDIT: Facepalm here too. I just examined the motor / transom relation and at level trim, the AV plate is actually below the bottom of the transom, looks like about 3/4" or so. I'd have to properly level the boat and motor to say for sure, but it's definitely below.

 

[steelespike]

Re: Help me minimize ventilation

Yeh I think a 12" 4 blade will help.Just to remind It shoul resist venting,have better hole shot, get a load up better,
stay on plane better at reduced throttle and handle better around the dock, probably will lose a little top end.

 

[whosmatt]

Re: Help me minimize ventilation

Yeh I think a 12" 4 blade will help.Just to remind It shoul resist venting,have better hole shot, get a load up better,
stay on plane better at reduced throttle and handle better around the dock, probably will lose a little top end.

Any recommendations if I decide to go with a stainless 3 blade? What should I look for?

 

[Jlawsen]

Re: Help me minimize ventilation

Your skiff is going to ride up in the stern with a following sea and you're going to ventilate if you can't get the engine back a little further. There are two solutions that I see. First and most preferable would be an off-shore platform but that can get expensive. Second would be a hydraulic jack plate. CMC makes a sweet one that will let you raise or lower a full 6". The standard plate will give you a 5" offset but you can get them with more. CMC is just one of the brands out there, have a look around.

 

[whosmatt]

Re: Help me minimize ventilation

Let's focus on the prop and performance in the bay for now, then. Getting a set back on the outboard can come after I get the most out of my current setup with a better prop.

 

[steelespike]

Re: Help me minimize ventilation

Any prop change is a bit of a gamble;when you add in a material change it introduces more unknowns.
Stick with a 4 blade 12" pitch prop.If there is a diameter choice go for the larger one.

 

[Texasmark]

Re: Help me minimize ventilation

Leave everything where it is except the prop. Get yourself a high performance SS prop with cupping and amaze yourself with the difference in performance. To me, with the numbers you posted, I would think that a SS of the same approximate diameter and exact pitch would not only solve your ventilation problem but may gain you a couple of mph. The SS has thin blades which provide more mph for everything else equal. The cupping loads the engine somewhat like if you upped your pitch an inch but what it does is to grab the water in turns and following seas to prevent the problem you mentioned. Additionally, if following seas are a problem, tuck in your engine which will help to solve the problem.

I think your numbers are stellar. Do as I mentioned and you may be pleasantly surprised.....you surely won't be disappointed.

Mark