1985 mercruiser 3.0 140hp, prop sizing?

[aski]

ok so I bought this boat a few years ago and it didnt run. After a full tune up, pertronix electronic ignition, carb rebuild, alternator, and a new fuel pump I finally have it running top notch. Problem is it didn't come with a prop on it and I think our local marina had one he was itching to sell... With my wife and I in the boat it will go decently fast, but the rpms never really go over 3250. With 4 to 6 people in it and coolers and whatnot, it wont get out of its own way and about 3000rpm is the best I can do. it has a 14.5x19 prop on it. Will going to a 14.5x16 prop make my issue any better? I know the motor needs to be somewhere around 4400 rpm at wot, and by not reaching that it is seriously low on power. just seeing if anyone has any prop sizing info they can help me with. Its a 17' open bow with the merc 140 alpha one lower end. Weighs somewhere between 1500 and 2000 lbs, fiberglass, deep v. im not looking to break any land speed records here but I know that my friends 90hp fish and ski with 6 people on it smokes my 140hp with 2 people on it.... I am a mechanic and ive been through everything including timing and advance and carb setup. I am completely convinced that it has to be a prop issue at this point, there is nothing left to do to the engine. any help would be appreciated. Thanks!

 

[Chris1956]

You want to choose a prop that will get close to 4400 RPM at WOT with a normal load. So lets start with your 3250 with wife and you. If you dropped 4" or pitch, the RPMs would jump up 1200, in theory. Of course, you may get a couple hundred RPMs more, because you are better in the power band. So the 16P prop is certainly worth trying...

 

[jimbo_jwc]

December 10th, 2014, 11:38 AM
Prop results after acquiring 3 different one's over last winter and using on 1987 40hr 3.0 L MCM Mercruiser w/rebuilt original lower unit on 19'6 ' Cuddy Cabin boat .
Original Prop Mercury 3 blade 14.5 x 17 WOT lightly loaded 5000+ rpm
Mercury Prop 3 blade 14.5 x 19 WOT medium loaded 4200 rpm 36mph GPS
Solas? Prop 4 blade 14.25 x 18 WOT lightly loaded 5000+ lightly loaded
Mercury SS vented Prop 3 blade 14 x 19 heavily loaded 4600 rpm 39 mph GPS Is what my boat likes best .
Motor has had new plugs ,fuel pump,points ,carb rebuilt,gimble brg and motor temp at 145-150 running hard WOT over an hour .

Different brand of props and sizes is hit n miss because of blade size's profiles and boats characteristics .

 

[steelespike]

Selecting a prop is like picking a single gear in your car to do everything.Your motor is working hard like pulling a trailer up hill all the time in high gear.
A lightly loaded wot run get us the rpm and gps speed.remind us of the prop pitch.
The 3.0 is one of the easiest outfits to prop.Your 17 footer would typically use a 17".19" or 21"prop.With a 21" prop should run around 40 mph with reasonable hole shot.May struggle a little with water sports and a heavy load. A 19 would have better hole shot and higher wot rpm might over rev with a light load.
A 17" prop is usually suggested for heavy loads with water sports and will over rev with a light load.
There are 3.0 boats that manage a 23" prop but with more limitations than the 21.
The 21" prop is like overdrive in a car, 19 more like high gear and 17 so on.
You still have issues other than prop selection. Does the boat respond to up trim? Does it get up on the water or does it lay down like its stuck to the
water. Does it have a hydrofoil on the outdrive?
Does it handle strangely like the bow is steering it in a strong turn.Is the bottom clean and smooth? Some one may haver changed the outdrive.
You may want to confirm your gear ratio. 3.0 is usually about 1.98 to 2.00 to one. Motor to prop.
You may have to pull the plugs and rotate the motor by hand and count One turn of the motor to 1/2 turn of the prop and so on.
Being a 85 the hull may have a problem.Is the floor spongey,has it been rebuilt? Does the bottom seem uniform no humps or valleys?

 

[eavega]

I'm going to second steelspike's suspicion that something else is going on with your setup. I run a 1988 Mercruiser 3.0 L with Alpha 1 (its either a 1.94 or 1.98 gear ratio, manual says one thing, online charts say another) on my 17.5' Stingray bowrider which has an empty weight of about 1850 Lbs. My "speed prop" is a 20" pitch SST three-blade. With that prop, lightly loaded (i.e. about 500 Lbs of people and gear, not including 20 gallons of fuel) I can tease 42 MPH SOG @ 4400 RPM on a windless day on flat water. My watersports prop is a 17" Pitch aluminum 4-blade, which my engine can still spin to about 4500 RPM with a top speed of 38 MPH SOG.

Although every setup is different, the 3.0L should not have a problem spinning up a 17" pitch prop to proper 4200-2600 RPM on a smaller bowrider . I would think that dropping to a 16" pitch would put you in "underpropped" category unless you are seriously overloading the boat or as steelspike suggests in his last sentence of his post, there might be an issue with your hull (saturated foam, deformation of the hull).

 

[aski]

Wow, love all the suggestions guys, thanks a lot. So lightly loaded, my wife and I and a cooler the boat planes out just fine, rides on the water real nice, but never gets the rpms that it should.. Always feels like it wants to turn left but not so bad that it seems like a problem. The hull is straight, no humps or valleys, and very clean. The floors all strong and sturdy, it has never been replaced. What is this saturated foam deal? And how is it diagnosed and fixed if it is a problem? Maybe I still have carburetor issues?

 

[aski]

I should also mention, lower unit ratio is 1.98 with a hydrofoil. And when the boat is loaded, 4 people and lots of soda, it never gets on plane. Just plows through the water.

 

[Starcraft5834]

I would agree w Chris.. 16p should be examined... Im a 4 blade fan too by the way , you wont "just plow thru the water" with a 4 blade, assuming of course your engine health is reasonable

 

[steelespike]

I should also mention, lower unit ratio is 1.98 with a hydrofoil. And when the boat is loaded, 4 people and lots of soda, it never gets on plane. Just plows through the water.

Ok we'll assume no one changed the ratio. The hydrofoil probably helps the hole shot but costs some top end.It also may be causing the strange handling issue.
If you want a true indication of its best speed remove the foil and do another test run.Don't worry about the holes.
We need the numbers rpm and gps speed.Some cells have a gps app.A 16" prop will certainly help your performance but you need to find the problem.
There was a guy on the other day with a 3.0 with a bad lifter it was quiet but not lifting the valve essentially a 3 cylinder.
He took a lifter out of a neighbors junker and solved the problem.He intends to do a new cam and lifters.
Have you tried a cylinder drop test?
assuming an unlikely 4200 rpm and a 16" prop we get about 28 mph at wot.
Sometimes the lazy mans way to check for wet foam is to jack up the bow and leave the plug out and watch for
water dripping out the drain more or less continually.

 

[eavega]

What is this saturated foam deal? And how is it diagnosed and fixed if it is a problem?

So, you probably have flotation foam built into your hull, beneath your deck. I think its a CG rule that boats built after a certain year have closed-cell flotation foam that will keep it above water for a certain amount of time. Over time, should a leak develop on your deck (or in some cases due to the design of the boat) the foam remains in contact with water for a long time, begins to break down, and starts absorbing the water. It never dries out. the net result is added weight to your boat which can manifest in failure to get up on plane, or not sitting level in the water. It can also lead to structural rot if the foam is in contact with any wood in your hull. There is no solution to saturated flotation foam other than digging it out.

Two ways to diagnose; you can either drill inspection holes through your deck and check if foam/shavings come up wet or dark. Problem with that is that sometimes the foam is not where you inspect, so you may not find anything. Easier (in my opinion) is to take your boat to a truck scale. Weigh it on the trailer, unload your boat, and come back and weigh the empty trailer. The difference is your boat weight. If the foam is saturated it will manifest itself as a significant increase in weight. You need to know what your boat should actually weigh, though (that 500 Lb difference in your original estimate of the boat's weight is significant, as it is probably close to a third of the weight capacity of your boat).

Rgds

E

 

[aski]

Ok I think weighing the boat is likely the most accurate way of checking it. It does sit fairly level in the water given that the tank and engine are both behind center of the boat. I know the original owner that I bought it from, a good old friend of mine. The boat was stored covered and under a canopy since it was new so I can't imagine a water logging issue like that. And I've stored it under cover since I got it as well. The bilge never really gets wet even after a long day on it. And the lower unit was rebuilt to factory spec with mercruiser parts about 4 years ago, I have the paperwork on that. I have not tried a cylinder drop test yet but I did run a compression check on it when I was first getting it to run, I forget the actual number but it was solid and with no more than 5 psi variance down the line. I'm going to swap my 19 pitch for a 17, recheck my timing, and try running without the hydrofoil this weekend and see how she goes. I think that I'm so confused because I rarely have the same load in it from trip to trip, and we always have a lot of soda and ice aboard. I'm going to take it out by myself unloaded and see how it acts. The more I think about it the more I suspect my timing may be off as well, it acts like a car that someone tried to poor guy power add by turning the distributor without actually knowing how to do it. I've run into that before....

 

[fishrdan]

Suggestions so far are good, gear ratio, soaked foam. Are you sure the tach is accurate, and what is the boat's speed by GPS? I would also look into the carb opening all the way (adjustment) and making sure the ignition is getting full advance (no rusty/broken weights and springs). I'm guessing it's a points distributor, so I'd check the dwell also, along with the timing.

With my wife and I in the boat it will go decently fast, but the rpms never really go over 3250.... it has a 14.5x19 prop

If all is up to snuff, the boat should be running near it's WOT RPM with that light load, certainly not 1350RPM low.

Remove all the "stuff" from the boat and run it down to the truck scales to see what the boat/trailer weighs. I did this to an old 16' fiberglass boat I used to own and I was shocked at what it weighed, somewhere around 3500#, lots of soaked foam and it now rides the waves out at the local land fill...

 

[Grub54891]

My 85 17' Larson mercruiser 140, 19" prop, ran 40 mph with wet foam, restoring as we speak. Just picked up a 17' 83 chaparral, mercruiser 120,same prop, runs 40 also.
Both bowriders. The chaparral does not seem to be waterlogged. Rpm's on both are at 4200 Rpm's. And my holeshot is pretty good. Even with 5 people onboard it would go 38-40 bit took a bit longer to get there loaded.

 

[steelespike]

While I believe what your gauges say you can't go 40 at 4200 with a 19" prop. Is the speed by gps?
If speed is by gps then very likely the tachs are wrong. Not unusual.

 

[Grub54891]

That was GPS on the Larson, haven't checked the chaparral yet with GPS
I shall check the tech this coming weekend just to be sure

 

[steelespike]

If we use 13% slip with a 19" prop at a gps 40 we get about 5000 rpm. Slip on your little boats is likely 10 to 15%
(easy to push) Slip defines your props efficiency through a liquid.

 

[Grub54891]

Yup,I get it. I will check the tach.