is my boat just slow

[milkman250]

I have an 88 vip valiant 18ft with omc 4.3.
I am currently running a 13x19 aluminium prop, I turn it 4600 - 4700 rpms (wot) and by gps I run 38 mph. So many people with similar set ups are running high 40s to mid 50s, is my boat extremely heavy or maybe a cheap quality prop...thanks for any advice, oh yea that's trimmed up as far as I can before prop slip

 

[Joshua Nichols]

Re: is my boat just slow

Waterlogged hull? That's slow... Should be doing mid 40's

 

[craze1cars]

Re: is my boat just slow

Interesting random comparo discussion...
http://forums.iboats.com/showthread.php?t=261192&page=1

You can use a prop slip calculator to determine for absolute fact whether or not your prop is the problem. The only info you're missing for a slip calculation is the outdrive gear ratio. What is it?

If the calculation says you're slipping bad, then go prop shopping. But if your slip is within acceptable range, then you need to look away from the prop and get into the rest of the powertrain to find the problem.

Do you know for a fact that your tach is correct? This is very important. The fact that you are providing GPS speed is very good and helpful...

Comprehensive tuneups do wonders. When was the last time your 23-year-old rig was treated to a compression check, set the base timing, check the advance timing, new distributor cap & rotor, new plugs, new plug wires, carb clean & tune, throttle cable adjustment, flame arrestor cleaning, engine/outdrive alignment, gimbal bearing check...etc? Systematically attack ALL of the above. I have found sevearl old 4.3L boats that simply are not opening throttle plates all the way...or have gunked up flame arrestors that are essentially running with a choke on 100% of the time ultra-rich...or have base timing retarded by 5 degrees...or a lack of timing advance due to corroded distributor...or partially plugged main jets forcing it to run way too lean...or are flat out just running on 5 cylinders...does your boat have points or some type of electronic ignition? What do your plugs look like in terms of fuel mixture/color?

Generally when I find and fix the problem(s) whatever it is, the boat will gain 5 to 10 mph with ease...

Waterlogged hull will indeed slow a boat down...but by how much? In my experience not much at all. At about 8 pounds per gallon you'd need to have 23 GALLONS of water on board to equal the weight of ONE average 185 pound person. I HIGHLY doubt your wood and foam can hold that many gallons and keep it hidden...and even if it can, does adding ONE single 185 pound person to anybody's I/O runabout slow it down by a full 8 to 10 mph? No. Adding 5 or 6 people might however. So do you think you have 125+ gallons of "hidden" water on board? Again, No.

So look elsewhere for a possible performance problem. My last 18 foot runabout of your vintage (Rinker) was equipped with a 3.0L, it actually WAS waterlogged and needed transom/stringer work, and it still ran 38 mph top speed on GPS with 4 people on board. So as you already suspect, yours should indeed do a bit better than it does now.

 

[milkman250]

Re: is my boat just slow

yes a complete tune up has been done, boat runs smooth and strong all the way though the rpm range. Tach works properly, used my dwell meter to verify. I think my gear ratio is 1.68 or 1.86, I picked up a 13 7/8 x19 ss prop today with bigger blades, gonna try it tommorro. This prop looks twice the size of the aluminum 19 I have now, what kind of differcence could I expect to see with a bigger 19 if any at all.

 

[ussoldier_1984]

Re: is my boat just slow

I have a similar setup a 95 18 foot maxum with a 4.3 lx merc, I can run about 60 mph with mine using a 14.2 X 23 pitch prop by solas.

 

[craze1cars]

Re: is my boat just slow

Well, if you have 1.68 outdrive you have 24% slip. Not acceptable. If you have 1.86 outdrive you have 16% slip. Not great, but substnatially better than 24. Is your prop chewed up?

Still need to know your exact outdrive ratio to determine whether you have an excessive prop slip problem, or another problem.

Since you have opportunity to try another prop, especially a stainless, by all means try it out...absolutely nothing to lose and will help you identify where your problem lies. I'll guess your holeshot worsens, your RPMS come down noticeably, and your top speed stays similar...and then you'll wish you had a 17 pitch SS instead of a 19. But obviously that's only a guess....

Report your findings.

 

[milkman250]

Re: is my boat just slow

Got some results 1st prop was a ss viper 19, at 4600 rpm I was going 37. The next prop was a ss 19 with little blades and at 4600 did 40, I did notice rpms getting up around 4800 4900, I have a 4.3 omc with a 3.0 out drive, I think my gear ratio is 1.86. Would this be the reason of slow speed with these props having a lower gear ratio. So if I went to a 21 pitch I would lose 400 rpm, but would their be a gain or loss of speed?

 

[craze1cars]

Re: is my boat just slow

The next prop was a ss 19 with little blades and at 4600 did 40, I did notice rpms getting up around 4800 4900,

???? I'm having a hard time making sense of this result.

So when you had this particular prop, was your actual max top speed 40 mph? If yes, was the engine turning 4,600, 4800, or 4900?

To answer your other question regarding whether bigger pitch would get you bigger speed, again just play with a slip calculator to roughly predict your changes in speed, etc. But you will need to have accurate RPM data all around in order for the calculators to work. Here's one of many:
http://www.mercuryracing.com/propellers/propslipcalculator.php

 

[milkman250]

Re: is my boat just slow

The 40 mph was at 4600, that is max rpm for my engine, I already turned gps off when running those other rpms, it started to rain and we were hammering down on it to get back to ramp. The higher rpms could have been to trim up to high also, I was throwing bit of a roost

 

[craze1cars]

Re: is my boat just slow

So are you saying at 40 mph/4600 rpm you were NOT at wide open throttle? If that's the case, then by all means you are under-propped, and would benefit from higher pitch to allow you to open the engine up fully.

If you already were at WOT running that speed and RPM, I doubt you would gain much if anything from going to a 21, but you would harm your holeshot a little bit.

As for running 4,800/4,900 "throwing a roost" is useless data. This indeed means you were trimmed up too high...which tells nothing...other than you were blowing the prop out of the water and putting extreme wear to the prop, outdrive, u-joints, and engine all at the same time...

The best I can tell with data provided: 4600, 1.86, 40mph, 19 pitch....that prop got you 10% slip, which is good, so that is the better designed prop for your boat, compared to the other one that would only get you 37 mph (16% slip) at the same RPM. What about that 37 mph test?? Were you at WOT on that test, or were you still at partial throttle when you hit 4,600?

A 21 pitch of the same design running 4,200 (which is nothing but a guess) would get you probably about 40.5 mph. However, if such a prop allows you to run WOT without over-revving, it would probably improve your top speed as well.

 

[milkman250]

Re: is my boat just slow

Ok as I just figured out at the prop shop, that prop I used that went 40 mph was a 21 pitch, we had to take a brush to the inner hub to see it, it had wrong tag on it. So now with this info my slip is at 19%. He gave me a deferent 21 with longer blades and a completely different design. Its a ballistic and blades look like the end are cut straight instead of rounded. I will try to get better readings and post in a couple of hours, thanks for your help. how much slip is considered good?

 

[craze1cars]

Re: is my boat just slow

It entirely depends on the boat's hull design and the speed you're running. But generically speaking at top speed a good prop should calculate slip at 15% or a bit less on a runabout of that size.

I don't think this has already been asked, but is the bottom of your boat clean? This makes a HUGE difference in top speed, especially if you have hard water stains or algea on the bottom....such gunk causes speed to go down and slip to go up very noticably. Algea covered hulls can slow a runabout down by a full 10 mph or more.

Note that when you are playing with prop slip calculators, most will advise you to add 1" of pitch when doing the calculations if your prop is "cupped".

I do believe most Ballistics are considered pretty high performance and are generally considered to be cupped props. So you probably will want to call this one you are testing a 22" prop when you have a chance to run it thru the calculations.

Sounds like a good prop to test. Report back. Make sure you are trimming the motor for absolute top speed....you are NOT trying to trim for top RPM. A GPS helps this dramatically because you can see 0.2 mph increments. If you begin to notice your RPMS start to creep up while your speed starts to creep down while running full out, that means you've trimmed it up just a bit too high....so run the trim back down a little bit, so the prop re-bites the water as hard as possible and rpms creep down a hair and your speed creeps up again...this is the sweet spot where it's trimmed as high as possible without pulling in ANY bubbles of air which create prop slip...it's a subtle "feel" thing, and THAT is the exact RPM we are looking for in these calculations.

 

[milkman250]

Re: is my boat just slow

Well I'm on the lake as we speak, and the hull is clean. Well I guess I'm stuck at 40 mph. 40 mph at 4550 rpm, iv played with trim and 40 is the best I can get, my gps doesn't have decimal reading, using cell phone buts its dead on while using in vehicle

 

[milkman250]

Re: is my boat just slow

Ok I hit 41 but not steady, it was back and forth 40 41.

 

[craze1cars]

Re: is my boat just slow

Well....maybe we revert back to the title of your thread...it's just slow?

That's still a bit more slip than we'd normally see in an I/O of that size, but maybe just a function of the VIP Valiant's hull design and outdrive positioning. Comaring runabouts of different brands and designs is useless...hull design makes for 10 mph differences easily, and you're stuck with what you've got regardless. Looking at that link I posted earlier, there were 3 people who responded with their top speeds who actually owned 4.3 VIP Valients of your boat's late 80's vintage. They reported 43, 45, and 52 mph. None of them specified that these were GPS speeds.

So if you're getting 41 gps, you're in line with the first two. As for the guy who claimed 52? Fluke? Liar? Other mods done to boat? I have no idea....

So 41 just might be about all she has. The good news is you've gained 3 mph over your original post, so apparently this prop is better. You have nothing to lose by experimenting with more props, but that one has you in a pretty happy RPM range so maybe it's good. I'm not familiar with Ballistic in general. Maybe others will chime in...frankly I'm pretty much out of ideas for ya...

 

[milkman250]

Re: is my boat just slow

Thanks for all your input, I know its not a race boat, but at the same time I want to get all I can out of it. Iv done lots of work to the boat and its a lot better than before...24mph lol, I'm happy with it. If I come up with some significant changes as far as speed or prop, I will post it here.

 

[Mac525]

Re: is my boat just slow

Are you giving it wide open throttle or are you backing out of it to keep the revs within spec

 

[milkman250]

Re: is my boat just slow

Wide open throttle, holding none back

 

[Mac525]

Re: is my boat just slow

I cant find any spec's for an 1988 omc 190 hp and any spec.s I do find over 175hp are for 5.0 liter v 8s is this a different motor or am I missing something

 

[milkman250]

Re: is my boat just slow

Well guys I know this topic is a year old, but I have a update, last week I took the doel fins off of drive and picked up 4mph and 150 to 200 rpms, now boat is running 4700 at 45.5 mph, the fins must put a lot of drag on my boat