Boat Builder Central - Builder Forums

Planning on a powertech 4-blade. Got a 40hp yamaha. Any help with a starting pitch would be appreciated.

Talk to Ron at Central Florida Props...

http://www.centralfloridaprops.com/

(321) 453-0017

He carries, repairs, & customizes Powertech's as well as most others. He also has decent prices, a great reputation, and expert knowledge of shallow running prop performance.

Called Ron and he fixed me up with a 4-blade powertec.

First off, what size prop are you running right now? If you are runing 13" pitch go with the power tech 4 blade tripple cup. This is a hell of aprop and will get you one hell of a hole shot plus should stop your cavitation. It has a lot of meat on it and you can run through the oyster bars and keep going. The number is PFS 4R12POM 140. Get a price from Coastal and I will beat their price . They list it from power tech direct around $524. I might be off $10.00 one way or the other. I can save you around $100 off list price. Send me e-mail if interested.
oken@bellsouth.net

Thanks for the information.

I have already ordered one from Ron. 4-blade powertec and he is going to put extra cup in it. Don't know the part number. I'll try it and get back to you.

Looks like everyone has the same problem with the boat cavitation. Where I am, with all the rocks, being able to get the prop up in the tunnel is worth a little aggravation with cavitation.

If it is really bad I'll probably go with a larger motor and a jet foot. It seem that Springhillsteve doesn't have these problems with his jet.

Oh well I only bought the boat as a beater boat for the rivers and creeks around the St. Marks and Aucilla rivers. It will be fine for where I want to use it and it will allow me to keep my bay boat out of these destructive environments.

I don't know if you saw my reply to your other post, but you should try venting the tunnel before you go to the expense of repowering. If it doesn't work any better you can always plug it.

I'm running a little 4 blade PowerTech on my GF-16 as well.

Tim

I built the Ph15 all foam version for a customer in Jacksonville. Running a 50 HP E-Tech engine with a power tech 4 blade prop. Running straight out on flat calm water it would lose bite and cavitate. Talked to Jeff Whidden at Power Tech and explained the problem This is how the tripple cup 4 blade came into play. Ordered prop one size smaller and it did the job. Jumps out of the hole quicker and runs out without cavitation any more. Actually picked up 2 more miles per hour. Great prop and has lots of meat in it. Captain Hendersen uses this prop for his Micro Draft skiffs. We used his boat as a guide when Jacques and I designed the XF20.

I'll check with Ron tomorrow and see exactly which Powertec he has ordered. If he hasn't got the right one ordered I'll try to change it. Hope he has the right one or that I have caught it in time.

What do you mean by one size smaller.

I think mine is a 10.5" dia. 13" pitch.

Usually when you run a 4 blade you have to downsize one size. You are turning a lot of wheel which causes stress on your engine. In order to keep the RPM's up you drop one size. In your case go to a 12" pitch.

10.5 X 12

When you built your Xf 20 did you keep it light as the plans show or did you add floor and extra goodies weight wise?40 will prob. work as plans drawn. Will not be fast but will keep you shallow running.What type of jack plate did you end up with. Hope not a Detwiller. they weigh in around 65#. Bobs weigh in at 35#

I bought a boat someone else built. It has a sole. I just installed brass screw in drain plugs on each side of the tunnel. Got over ten gallons of water out of each side. Getting that weight out should really help performance. I'll probably cut out the sole this spring after the creek redfish season ends. It was foamed and I am really curious what I will find.

Going to try out the new powertech four blade and hydraulic jackplate this weekend. Can't wait to see how it performs.

Was the inside of the boat waterlogged? Richard

Doesn't seem to be. Won't really know until I cut that sole out.

After running over all the rocks and oyster bars this boat will see between now and spring I'll probably cut out the sole and have to put a few more layers of glass on the bottom in my spring cleaning.

Its just what I wanted a boat to use and abuse. Whatever I tear up can easily be fixed.

Sounds good this type of boat is made to be used and abused. Jack up the sole and make it self bailing if it is not already. Tight lines, Richard

Redfish55
I just got done testing my XF20 after venting the tunnel just as timoub007 suggested. I went from 22 mph to 27.3 mph with a 25 yamaha with NO cavitation. I'm sure I will get more when I prop it out because the rpm's were wound out. The boat also rode quite a bit higher in the water. Getting rid of that suction was the key. Thanks Tim!
Pita

Pita,

Glad it worked out nicely for you. You got more improvement that I did, that is awesome. That is a 24% increase if I did the quick math correctly.

Good deal!

Tim

I completed an xf20 build this past summer and have gotten alot of use out of the boat. I beefed it up with a stringer system, sole, gunnels, decks, lids, etc. I run out of the St. Lucie inlet in Stuart and have to share the tight inlet with alot of boat traffic as well as some rough seas on the outside. My boat is heavy but still floats in ankle deep water and can get in a few local shallow haunts. For power I have a 60hp 4 stroke suzuki on a bob's jack plate wtih 4 blade composite prop (13 dia w/ 19 pitch) (Composite was a mistake - flexes too much and blades get chewed up in the sand). Also - I would go bigger on power next time (70-90), I'm running low 20's now.

I had issues with cavitation when she first hit the water so I put on a cav plate from Bob's. It helped but I still have some cav after planing.

Once I install a tach I wanted to start prop hunting and bite the bullet with stainless. Two posts on this site addressed my issue, but I wanted get some follow up.

Ken - you mentioned a four bladde power tech - for the description of my xf (heavy) what would be your recommendation in prop size and pitch.

timoub007 - yours is the first I heard of venting the tunnel, which looks lke it produced results. can you provide a little more detail on this, and are there any cons to venting.

Here's a link to my finished xf.


http://gallery.bateau2.com/displayimage.php?pos=-14848

Chris,

Congrats on a nice boat. Click on the link in my sig to read about how I vented the tunnel in my GF-16T. If you have specific questions post them or email me.

If you call PowerTech, they are very good about taking what data you can give them and recommending a prop that should be correct. Good folks.

T

You say venting the tunnel. Describe what you are talking about. I have an idea but am not sure. How did you do it.

I looked up your vent. It appears that it is located at the front of the tunnel (towards the bow) - is this correct?

I don't know much about the tunnel design, but does this vetn only air or do you get some water back in the boat as well?

Ken, yes venting the tunnel.

With the vent installed in the tunnel I saw a dramatic performance increase. On my initial test run I had a moderate load and with the vent open we could reach over 24 mph, almost 25. When I put my hand over the vent to block it off, the speed fell to 21.5 mph w/o changing anything else. Also the RPM's increased as the prop started to slip in the turbulent water when the vent was blocked. As I removed my hand the RPM's would slowly drop and the speed picked up as the prop started biting again. There was a lot of suction at the top of the vent when my hand was next to it. With my leg next to it, the suction was pulling my leg hair into the tube. Hard to judge vacuum, but it was substantial!

The vent was simply a 1 1/2 inch conduit threaded collar and a long sweep ell. I cut a 2 inch hole in the front of the tunnel and sealed the collar in with 5200. I have designed something more permanent and less obtrusive in my new tunnel using a SS through-hull fitting.

Here are two pics of my test vent in the old tunnel, which is just a more narrow version of the XF tunnel.




Chris,

The vent is at the front of the tunnel just as it starts to make the slope upwards. By putting it here you break the suction that is being formed by allowing air to be sucked in. The vent will puke back water on you when you drop off plane and the hull settles down in the water. To prevent this, you can build a baffle and/or run a piece of flex tubing over the side to give the water a place to go. I have built a baffle on my new one which will also help with water flow.

The new tunnel changes width from the front to the back to reduce drag. The purpose of the tunnel is simply to redirect water and I am trying to limit the amount of turbulence induced into the water. The old tunnel seemed to compress the water with its design, as Jacques mentions it will do in the XF-20 study plans. This creates the vacuum that I relieved by putting in the vent (venturi effect). I am attempting to redirect the water w/o compressing it by allowing a greater volume of water in the throat with the wider mouth. I calculated the volume at the throat and at the exit. While I was not willing to give up enough planing area at the front to make these volumes equal, I am closer than the original design.

A local aluminum fabricator with much experience and success with tunnel design on this size & style of boat recommended this design. He has built one similar with great results.

If there are any other questions I'd be happy to answer them.
Tim

Ran my boat with the new powertech prop today. No cavitation. 10" pitch four blade. It has a ridge on the prop edge. must really make it bite.

I ran through some really shallow water today. Just wanted to see what it would handle. I did run hard aground I mean the boat was stuck. Had to wait about 30 minutes for the tide to come in before I could push it over to a little deeper trench. When I got out to push the water was just over my ankles.

Caught about 30 redfish and a dozen nice trout. Would have never gotten where these fish were without this boat. The only other boat I saw was an airboat.

What do I need to do to vent my tunnel? Drill a hole and put a piece of 3/4" pvc in it and seal with 5200? Do you need to put some sort of valve on it to control the amount of venting you get?

I'm running about 20 mph. Sure would like to get another 5.

I don't know if 3/4" would be large enough, but it would be a good starting point. You could always go slightly bigger if needed.

As far as a valve goes, some boats have them and some don't. If you decide to put one on you should have it closed when getting on plane so that the prop will slip a little more and get the RPM's up. Once you're up on plane and picking up speed, open the valve to relieve the vacuum. If you think about it, just prior to coming off plane you can close the valve to prevent water from puking back up the vent. This would eliminate the need to run a hose over the transom or side.

T

Very interesting. I thought I have been around a lot of this. This is a new one for me. Good info.

Thanks for the information - as I currently use the space above my tunnel for tackle storage I think I will opt for a discharge hose through the side.

I'll let you know if it helps

I would sure like to see what Jacques had to say about that vent!

Rog...

Rog,

He breifly discussed it with us in this thread last week, but I didn't get a reply if he read the test results on my GF-16. If I can't claim the free plans, maybe he'll give them to Pita since he vented his XF-20.

http://gallery.bateau2.com/forum/viewto ... 5d82face07

Tim