22 galvanized boat with twin Merc 150 props?

[Bartboat]

Hi Folks, some of you were advising me late last summer on prop ideas for my heavy (5800lbs) self built boat I was rigging with twin new Merc 150 4 strokes. I found a Vensura 21 lh on eBay to at least get some test results. The 19rh Vensura pulled 5300rpm while the 21lh came in at 4900rpm @ 44 mph on gps. Merc says rpm should be 5000-5800. Question is, If I switch from the 4 blade to the three blade Enertia 19 what would your guess be on the rpm & speed.
I had a bit a swell to contend with and the motors still need to be broken in too. Also, I may be able to raise them another inch up on the bracket, and I'm thinking its capable of 47 mph with the existing set up? Boat can be seen on Youtube BartDavis100. yeah, i know its ugly.
Thanks for all your advice in the past. Bart

 

[gonzo]

The amount of blades won't change the RPMs unless there is cavitation from overloading the blades.

 

[Bartboat]

Thanks Gonzo! Motors are brand new and not broke in. Will I possibly see a slight rise rpm when they break in? I'm now thinking that I will swap in a 19 and that motor can see 5300 like its twin. Maybe both will go to 5400 as the one motor will run freer?

 

[FishStretcher]

It is commonly reported that 4 blade props can have more drag, but are smoother. Also, raising until the anti ventilation is above the water on plane is usually good for a few hundred RPM.

For older boats designed for light 2 stroke power, refitting with 4 blade" stern lifting" props can help when you are on plane. This might or might not be your case. I don't know the vensura prop, but it looks like a 17 P 4 blade might give great hole shot and same top speed. Assuming your motor height is high enough. If it needs a lift, then the 19 is probably closer. Or maybe a 3 blade 19P, which might be a bit faster. If you just hit the rev limiter at WOT when heavy, that isn't a bad compromise.

If you are close, you can often have cup added/ removed from the props at a local shop to get a final setup.

It is said that it you can't hit max RPM, you have too much prop, and risk a shorter engine life.

I know my F100 will run faster with less pitch and holeshot is much better. I have a 4 blade stern lifter on an old boat designed for light 2 stroke power. I run $110 Solas Amita props in aluminum. I may swap to stainless, now that I have a pitch and blade shape I like.

Guys with brackets often like stern lifting props as there is no planing hull adjacent to the engines to support them. Maybe this would be your case, too.

 

[gonzo]

The rake of the blade has a significant effect on the trim, as you say. The amount of blades alone is not enough to make a change if the pitch, diameter and blade area remain the same.

 

[Bartboat]

Hello Gonzo and Fishstretcher! Great info guys. Found a guru who had tried 24 different props on his Whaler. He was kind and patient to share his knowledge, although my boat's performance perplexed him at first. What comes down to for me is individual preference. Do you want a good ride? Good docking characteristics? Good cruise speed GPH? Or do I want to go 50? So, we went with Mirage 17's and at 5500 rpm hit 48.1 mph on the GPS. The ride is way better than the Vensura props. Much softer and lighter feeling. I have the AV plate riding just above the surface. I'm very pleased and after the motors are broken in may even hit fifty some day especially if I take the Doel-fins off. The Merc testing page shows several boats in our weight and configuration and we are within an mile an hour in some cases or faster yet were heavier. Not bad for a old design or home made.

 

[dinoa]

To hit fifty from this point on:
lighten boat
trim out until ventilation then back in just a touch
consider a transom jack
redistribute weight aft until onset of porpoising
polish prop
clean bottom
all above except keeping prop/bottom clean may adversely effect other aspects of handling/performance.

Dino

 

[Bartboat]

I forgot to mention that when we hit 48 we were mostly full of fuel. We went out camping for a long weekend so we weren't empty either. Wish I could get those barnacles off that are between the bunks and the hull!

 

[nukisen]

Hi Bartboat!
You typed "yeah, i know its ugly."

I want to say. Dont scale down yourself and the work you have done.
For the first I think most of us in here understand that the functionality was the most important when you designed the bracket. Well, all who wants can see that the bracket does its job.

To chase speed maybe air lubrication could be something for you.
Pipes through the bottom that makes small bubbles decreases the resistant.
A friend of mine did also sharpen the prop blades and gained a knot or two.

Else 48 mph is not bad either.
Feels quite fast.

 

[Bartboat]

Hi Nukisen,
Thanks very much for the kind words from Sweden! Funny you should mention air induction. When the hull was hot dipped galvanized one side at a time the hull developed warbles all over. I think this has created not unlike the golf balls dimples for reduction of friction. The hull only rides on the high spots and I think air is trapped in some of the lows. I was a bit worried at our new speeds we may see some issues with what they call hook and rocker on production boats, but it all worked out. The hulls heavy, but the superstructure is all composite. It's so stable, that running in a beam sea is amazingly comfortable.
Do you folks have a number of steel boats? Seems I've seen a lot of pictures of them up there.
I sharpened the blades once on our catamaran power/sailboat and it caused a lot of cavitation. Friend told me to put the blunt edge back on which corrected this. Every boat is different. Boat use to have twin 7hp VOLVOs in but after 20 years I put twin 70hp Mercs on it. Not many sailboats you can waterski behind! It's on the YouTube site too if your interested. Bart

 

[nukisen]

Actually I have participated in building passenger and roro hulls and maybe you have seen some of my pictures in here. 200 meters long and hull weight about 10 000 - 15 000 tonnage before mounting fittings and interior.

I have not designed the alurex of Sweden Tuna 18 but participated in the documentation and the owners manual to get the boat certified CE also corporated with IMCI to get a serious certification.

Its a good friend of mine who have built this aluboat wich is designed by a canadian boatdesigner. He have made a very good design and the boat performs very good in the element of water.
Also he is in here at boatdesign.net but I am not sure.

When we tested the Tuna 18 we where a staff of 4 persons and full tank (220 litres). It performed 37 knots with a 150 hp suzuki in behind.
No air induct or shaoed prop on this one. The stability is very good and we do have quite sharp sea when the weather is rough as the channel between Sweden and Denmark makes the wavetops narrow. In nice weather the water here can also be very calm and makes boating very comfortable.

 

[Bartboat]

Very nice, lots of freeboard. Wish I had a bit more. I really like the edges on the console on the near boat. We really debated about using aluminum. My high school teacher thought I might not be ready for it in the time frame we had. I was pretty pushed to get the hull and trailer done my senior year as it was. Great picture and here I though you guys wore skis all the time. Wearing shorts no less!
I did finally build some stuff in aluminum and will probably build a new aluminum trailer for this boat. I built a 80' ramp 15 years ago. It has wheels on both ends and is a cantilever design that reduces the weight on the float.