Trying to get a little more speed with tabs and hydrofoil

[geneseo1911]

Re: Trying to get a little more speed with tabs and hydrofoil

I'll jump in here and ask the OP if he has done a comprehensive tuneup. My '82 Merc 260 seemed to run great, but I thought it should be a little faster. I went through the motor with a fine-tooth comb (at the prodding of the fine folks here), and discovered that the mechanical advance in the distributor wasn't working at all. I lubed it up and gained 7 MPH. Just because it seems to run and start well, doesn't necessarily mean it's making full power.

 

[BlkY2k]

Re: Trying to get a little more speed with tabs and hydrofoil

Id have to agree with the last couple of posts here. Make sure its running like it should and go from there. Not only that but I think you guys missed one important line back up towards the top. "I did the speed reading using an iPhone 3G. Not pitot, not GPS, but really just cell tower triangulation." Id like to think my boat does 42 mph but I know it doesnt. 31 clocked on gps and like glass conditions too.

 

[dockwrecker]

Re: Trying to get a little more speed with tabs and hydrofoil

4000 rpm is way low. Geneseo1911 is on to one of the first things I'd look at. Sticky advance could easily be the problem. I've got the same motor, 19P Mirage prop, Alpha 1 drive and a much heavier boat (3900 dry, more like 4700 loaded) that runs @ 6000 ft altitude and even before I changed out the gears to 1:62's it would pull 4200 rpm. At sea level the OP's boat should easily pull 4500 with what he's got. Ain't no way to hit 45-50m mph @ 4000 rpm.

 

[Mike Zee]

Re: Trying to get a little more speed with tabs and hydrofoil

I am betting he doesn't have foam in that boat.

With a boat that size, I would not get smart tabs. If you really think you need tabs, get helm adjustable ones.

A 19" 3 blade prop should be what he needs assuming the engine is in good shape. I personally, would make sure a tune up was done and fuel filters replaced before anything else was done.


So what were the compression numbers? You said "rock solid" but I would be interested in what you measured.

+1 Old 14x19 3 blade had my boat pushing 48 down hill. 5.7L. With just me. I put on a 4 blade for pulling the tube, my specs and performance same as OP's. I had a 13 3/4 x 17 if you could keep it on plane it would go 50+ but rpm's way high. It was a spare, it wasn't on long. My boats a 24' cuddy.

 

[H20Rat]

Re: Trying to get a little more speed with tabs and hydrofoil

I never would have guessed that a 3 blade prop would be better for top speed. I'll look into a mirage with 17 pitch. I think the tabs will help with fuel mileage and lower planing speed, vice top speed.

4 blades have less slip, but you also have more metal to slice through the water. The drag the prop causes is what kills the speed. Look at the airplane world for good examples, especially old warbirds. They often had TALL landing gear because it would let them run a 2 or 3 blade prop, instead of a 4 blade. The idea was to absolutely minimize the number of blades within the constraints of what you could run for diameter.

Something to think about... The tip of a 14.5 inch prop spinning at 4000 rpm is traveling at 172 mph! Now it makes sense why you want to minimize the amount of metal spinning through the water.

 

[jumpjets]

Re: Trying to get a little more speed with tabs and hydrofoil

Hmmmm...... Tune up.

I replaced the plugs and wires, and ops checked the distributor to ensure all 8 cylinders were firing, but I didn't check the spark advance. Honestly, I wouldn't be surprised at all if it wasn't advancing.

I cleaned the air filter (flame arrestor), but didn't really go through the carb at all.

The engine had 150psi compression on all 8 cylinders plus or minus 5psi. I didn't do a leak down test.

When I bought the boat, it had a stainless 3 blade 23 pitch prop. I don't know if that was factory stock or not. I put the 19 pitch prop on because I thought it would increase the engine RPM, and it did. The 23 pitch prop was getting me up to 3700-3800rpm, and about 37-38mph. (roughly)

 

[bruceb58]

Re: Trying to get a little more speed with tabs and hydrofoil

Should also change your fuel filters.

You should check your timing at idle to make sure its spec. You likeley have the TB4 ignition so no distributor weights so you likely don't have an advance problem but worthwhile to look at it while you have the timing light on it.

Your 19p prop is in good condition right? No dings or bent tips? Is it cupped?

 

[geneseo1911]

Re: Trying to get a little more speed with tabs and hydrofoil

A 23 sounds way big to me, so I wouldn't think that could be factory, although I'm no expert and maybe there's a gear ratio issue I don't know about. FWIW, my boat is a 19' bowrider, but it is heavy for its length, in that same 3300-3500 lb range with gear & fuel. It currently runs a SS 21P , which does 52 MPH GPS at around 4200-4300 RPM.

 

[bruceb58]

Re: Trying to get a little more speed with tabs and hydrofoil

FWIW, my boat is a 19' bowrider, but it is heavy for its length, in that same 3300-3500 lb range with gear & fuel. It currently runs a SS 21P , which does 52 MPH GPS at around 4200-4300 RPM.

You have a 350 in a 19' bowrider?

 

[Philster]

Re: Trying to get a little more speed with tabs and hydrofoil

What we know:

She wore a 21p SS 3B prop out of the factory.

She was found with a 23p prop on her and you don't need a calculator of any type to know that is wrong all day and night.

She has a 19 on her now with 4-blades: This represents the correct step down to compensate for four blades. I think an SS, 19P, 3B prop is the way to go. I can't see how winding up as low as 17 in any shape or form is okay, even if the engine is 100% ok.

 

[TilliamWe]

Re: Trying to get a little more speed with tabs and hydrofoil

All good points here jumpjets, you have some trouble shooting to do. Good luck.

 

[joewithaboat]

Re: Trying to get a little more speed with tabs and hydrofoil

I have owned a boat of almost identical size and weight, same merc power. A 19p ss 3 blade worked well. Cant say that i remember exact top speed or rpm. Around 48 mph i think. The only reason i remember the prop is i still have it, sold the boat with an alum on it.

I would be taking a serious look at the ignition. I had a weak coil one time that limited rpm with no other indicators. Give a good look at the fuel system... including the check valve in the fuel line.

 

[jumpjets]

Re: Trying to get a little more speed with tabs and hydrofoil

Should also change your fuel filters.

You should check your timing at idle to make sure its spec. You likeley have the TB4 ignition so no distributor weights so you likely don't have an advance problem but worthwhile to look at it while you have the timing light on it.

Your 19p prop is in good condition right? No dings or bent tips? Is it cupped?

I don't know if my prop is cupped or not. I'm not too sure what that means. It's a brand new prop that I've only run a few times. It's in perfect condition. My alpha one outdrive had a bent prop shaft, so I bought a new outdrive and prop all in one. I did that last august.

I'm really leaning toward ignition problems based on what everyone has said. The coil looks like it's from the 1970's, and the distributor is a hot mess. The vacuum advance might not be hooked up correctly.

My first troubleshooting step is to weight the boat. It it weighs the correct ballpark figure, I'm going to check the timing. If the timing is good, I have no problem installing a new MSD marine distributor and coil, and setting an aggressive timing curve.

 

[bruceb58]

Re: Trying to get a little more speed with tabs and hydrofoil

My alpha one outdrive had a bent prop shaft, so I bought a new outdrive and prop all in one. I did that last august.

The vacuum advance might not be hooked up correctly.

If your distributor has a vacuum advance, you don't have a marine distributor and that could be your whole problem. The advance is completely different. Care to post a picture of it?

When the outdrive was switched, was the replacement outdrive the same ratio as the one it replaced?

 

[Philster]

Re: Trying to get a little more speed with tabs and hydrofoil

The gear ratio in the new drive is the same as the old drive?

 

[jumpjets]

Re: Trying to get a little more speed with tabs and hydrofoil

If your distributor has a vacuum advance, you don't have a marine distributor and that could be your whole problem. The advance is completely different. Care to post a picture of it?

When the outdrive was switched, was the replacement outdrive the same ratio as the one it replaced?

THis is the distributor that is on the boat now. I took these pictures almost a year ago, and I'm luck I found them online!

The gear ratio in the new drive is the same as the old drive?

I think the old drive was a 1.50 ratio, and the new one is a 1.47 ratio. They are pretty much the same.

 

[bruceb58]

Re: Trying to get a little more speed with tabs and hydrofoil

Looks right! Not sure why you were mentioning vacuum advance.

 

[jumpjets]

Re: Trying to get a little more speed with tabs and hydrofoil

Looks right! Not sure why you were mentioning vacuum advance.

There is a vac advance nipple on the cap that basically goes to nothing. I was assuming it was supposed to be used for something.

http://www.fquick.com/garages/viewgallery.php?albumid=3&carid=27298&page=1

This link shows a brief picture-story of all the work I've done to the boat. It is by no means comprehensive, but you can get a good idea of the rot that I initially fixed.

 

[bruceb58]

Re: Trying to get a little more speed with tabs and hydrofoil

There is a vac advance nipple on the cap that basically goes to nothing. I was assuming it was supposed to be used for something.

A vacuum advance would be a diaphragm off the distributor housing. What you are seeing is likely a vent.

 

[joewithaboat]

Re: Trying to get a little more speed with tabs and hydrofoil

That nipple is a vent it should be left open!