acceleration takes a looooooong time - prop related?

[mtfoyre]

Yesterday was only my second trip to the lake with my boat. The first trip ended w/ a gasket failure and 900 dollars worth of work to the sterndrive

Anyway, I noticed it takes a really long time to get to 4000 RPMs. Somewhere between 60 and 90 seconds. It immediately jumps from idle to 3000 RPMs; bow runs high and will start to plane around 3500 RPMs. It will literally take about 10 seconds for every additional 100 RPMs over 3k.

Once you get there though, it takes a lot of hard turning or extreme chop for the RPMs to drop and have to rebuild. It also isn't affected by dragging a tube...it will accelerate the same with or without a tow.

I'm wondering if this is (in part) because of the prop.

boat: 1967 Thompson 21'
weight: uncertain but I know it's a lot
engine: Mercruiser V-8 185hp (can't remember exact displacement)
recommended RPM: 4000 - 4400
sterndrive: ~1976 OMC
prop: 15" 13p (it isn't stainless...my mechanic told me it was bronze, I think)

I know that I have a lot of kinks to work out, but as I go I'm wondering if I should consider a different prop to get up to speed quicker.

Thanks in advance and I'll try to get any other info needed. I have the manuals at home but the boat is kept at a remote storage location.

 

[walleyehed]

Re: acceleration takes a looooooong time - prop related?

The fact that 4000RPM is all you can get tells me you are over-propped. Several conditions can be present to cause this...#1. obviously it's taking too long to plane out...a good sign we have either too much pitch, or bottom structure problems, maybe even a sterndrive unit not going all the way "IN" for take-off.
We need to know for sure the tach is accurate, but that's a good sized boat for a 185HP unit. I don't "think" the 13P is far off, but we don't know the health of this engine either.
If you can clear up some of these questions, maybe we can help ya out.

 

[mtfoyre]

Re: acceleration takes a looooooong time - prop related?

Bottom structure seems fine. No holes, scuffs or growth. The boat doesn't stay in the water. I've checked over it plenty of times.

I don't know what you mean by a sterndrive not going all the way IN for takeoff...I'm pretty new to boats.

Everything I've learned from the manuals indicates the motor is the largest offered on that model at the time. I believe it was 185 h.p. They guy I bought it from, however, said the couple he bought it from stated the motor had been swapped w/ a 327 c.i. unit, but I don't know how to identify that.

I have a friend that rebuilds a lot of carb'd motors coming to help me go over the motor items. I hope to know more about that side of things after I get his help.

You say it's taking too long to plane out. Do you mean timewise or 3500 RPM is too far into the range? Should it be planing at a much lower RPM?

P.S. I appreciate your help. When I get home I will try to provide more info from the manuals/paperwork I have.

 

[walleyehed]

Re: acceleration takes a looooooong time - prop related?

Taking too long time-wise...
3500 for that size boat is about right to plane out, it's just the time it takes after getting on top. It's not accelerating fast enough and we need to know why. we also need to know the correct RPM, "IF" that's what the tach is reading.
By saying "drive is all the way in" I mean the unit is trimmed all the way "down" until it stops moving...it should be tucked under just past parallel with boat bottom.

 

[Crownie2]

Re: acceleration takes a looooooong time - prop related?

Also - are you trimming out after you get on plane??

Bob

 

[mtfoyre]

Re: acceleration takes a looooooong time - prop related?

Crownie I don't have any trim adjustments. I can only assume it's because of how old the sterndrive is.

walleyehed, I didn't get a chance to pull out the papers last night, but I still have a question. When you say over-propped do you mean too much pitch, not enough, too large a diameter, etc.? Sorry, I just haven't caught on to all the lingo yet.

Also, I have a couple of aluminum props that came w/ the boat. One of them is a 15" 14p...I can't find the markings on the other. If they have minor dings and are VERY slightly banged up would they still be okay to try out? Just for FYI purposes...?

 

[tmh]

Re: acceleration takes a looooooong time - prop related?

no prop expert here, just some thing I've read in these forums and checkedinto myself that might help...

1. Smart Tabs (type of trim tab) should get you planing at lower rpms and maybe help accelleration, but not sure. Check 'em out.

2. My boat was a bit sluggish also - I'm re-doing the floor and just pulled out about 300 lbs of water-logged floatation foam! I KNOW getting THAT out will help performance next year! If you have original floor, etc maybe you're way overweight (not you, the boat!)

3. As Kenny said, sounds like a lot of boat for 185hp - again, a weight/power issue. I'm sure Kenny can get you proped right though, just don't expect a race boat!

Good luck.

 

[mtfoyre]

Re: acceleration takes a looooooong time - prop related?

tmh, thank you very much for the info.

1. I'd seen Smart Tabs mentioned but didn't realize they were a universal product. These look like they may help me anyway because steering is a full-time job at low speeds.

2. This is something I'd considered as part of my winter project. I want to do a lot of different things during the off-season to get the boat to 100% by next spring. I need to pull the floor anyway to inspect the tanks & lines.

3. Well, like I said 185 is what I believe the manual stated was the max. engine from production (it seems they even offered 4-cylinders with it...glad I don't have that!). I've been told it was replaced but until I get an expert's eye on it I won't know. Luckily I know that expert I just have to convince him to come over and help me go over some mechanical items. I'm great w/ fuel injected, computerized engines but carbs throw me for a loop.

I did find a line item on a receipt for over 3 grand but there's only a part number...no description and the place that did the work is no longer in business.

Thanks again, everyone. walleyehead I'll do my best to post specs from the manuals to help out.

 

[walleyehed]

Re: acceleration takes a looooooong time - prop related?

How do you lift the out-drive when you put the boat on the trailer?

 

[tmh]

Re: acceleration takes a looooooong time - prop related?

Some of the guys here can work wonders with the serial # from your engine. They cross-reference them somehow to tell you what you have. Maybe post in the engine forum if you can find the s/n.

 

[mtfoyre]

Re: acceleration takes a looooooong time - prop related?

How do you lift the out-drive when you put the boat on the trailer?

I have an electronic tilt mechanism.

 

[mtfoyre]

Re: acceleration takes a looooooong time - prop related?

Okay, I was wrong. 185 was the incorrect power rating for the OEM motor.

I'm at home and have dug out the books. There are only 2 V-8s listed...a 225 h.p. unit and 245 h.p. unit. In the diagrams they look identical. The only spec difference I can find between the two is one has an operating range of 4000-4400 and the other's range is 4200-4600.

Also, the outdrive on it now is not the same one in the book (my mechanic told me it was about 10 years newer than the boat so I already knew this). I don't know if OMC had similar gear ratios in both of these units, but the original was 1.16:1

This will take some time :$

 

[Crownie2]

Re: acceleration takes a looooooong time - prop related?

Is there any way you could post a few pics of the stern of the boat?

 

[mtfoyre]

Re: acceleration takes a looooooong time - prop related?

This is all I have.

 

[Crownie2]

Re: acceleration takes a looooooong time - prop related?

Thanks - this is indeed a heavy boat that is going to need everything to be in good shape to get good performance (engine tune, check for that water weight and good prop).

Bob

 

[tmh]

Re: acceleration takes a looooooong time - prop related?

I finally got most of the water-logged foam out of my 19 footer and I'm up to at least 400 lb. including foam, saturated carpets that never dry (under unexposed areas), wet wood, etc. There may be another 100 lb or so of foam, but it's a ton harder t get at the bow area floor, and it's not rotted.

YES, it's a big pain of a project, but imagine my excitement when I put it in the water next year 15-20% lighter! Most from the stern also. Add the Smart Tabs and I should be much happier - so will my engine be! I'd bet anything that IF your boat has floatation foam, it's soaked at the bottom. I first came to this forum looking for prop help also, but I was tld (correctly) that I should make sure other factors weren't the true problem first. Great advise!

 

[mtfoyre]

Re: acceleration takes a looooooong time - prop related?

Yeah, I agree there are many other things I need to go over. I just wanted to talk about the prop because I found the acceleration curve to be odd compared to most threads I've read on here.

How are you doing this? Are you pulling up the entire floor? I must admit the thought of taking the entire floor out scares me

 

[tmh]

Re: acceleration takes a looooooong time - prop related?

No, not the entire thing. First off, Most folks seem to recommend leaving 1-2" on the perimiter, leaving a "lip" to help secure the new floor section. So, I' doing that and then going about 8 ft starting from the stern and cutting across. If I were doing it over, I'd make sure it was either just under 8' or go closer to just under 12' as plywood comes in 4' widths if I recall.

My plan is pretty simple, I'll rough cut the new floor sections, leaving 1-2" overlap of the perimiter old floor. Then, I'll lay it in place, secure with a few screws, then cut my final cut through BOTH pieces, about 1 1/2 inch from the edge. This will ensure the new section fits perfectly with what I left from the old floor.

Then, antifreeze on any rott I didn't completely cut out (kiils the rot I hear), let it dry a while, then thinned epoxy on all newly exposed wood from old floor, stringers, etc to penetrate and strengthed old section (see CPES sold by "Rot Doctor" for the concept www.rotdoctor.com has lots of discussion, but I've read CPES is basically thinned epoxy product so I will just thin my own to save $$).

Epoxy both sides of new floor, especially edges, epoxy along stringers to hold floor in place, lay floor, weight down, maybe some screws, then glass/epoxy it all together and done.

Sounds like a ton of work, but most of the steps are really easy enough if you think about it. If you're in no hurry like me then time between steps for curing, etc. is no big deal.

btw, if I sound like I know anything don't be fooled! These are all steps I've absorbed from this and other forums from all the people who actually know something. I think my summary for you represents as close a "consensus" on how to do this job as best I could explain it.

Anyone care to chime in and set me straight on anything I missed please do so. I want to learn also as I am in-progress right now of the floor repair!

 

[mtfoyre]

Re: acceleration takes a looooooong time - prop related?

Sounds like a pretty sharp way to go about it. What about the foam? Are you replacing the wet stuff w/ something new or just leaving empty space where it used to be?

 

[mtfoyre]

Re: acceleration takes a looooooong time - prop related?

For everyone that helped me out; thank you. I took the boat back out yesterday to more closely monitor some things.

1) WOT gets me 4k RPMs. If the prop is the issue, it's just barely. I figure dropping pitch by 1 would get me right into the sweet spot of about 4400. The time to get to 4k did seem to improve over the last trip. Maybe I'm dealing with a tune/gunk issue here.

2) A good cruise speed seems to be around 3500 RPMs. The boat stays on plane and I figure running WOT 100% of the time can't be good.

3) When I was running WOT for a while I noticed that I'm running at a similar speed to the majority of the boats on the water (I even passed a couple!). I don't know my speed because I don't have GPS and the pitot for the speed-o disappeared 2 weeks ago on the water.

4) I'm going to use the off-season to take care of some things: clean carb, replace plugs, check timing, compression test, verify tach, water/fuel separator, check for waterlogged foam, etc.

5) Before I replace the prop I'll take care of all those things and also invest in some Trim Tabs. I spoke w/ my mechanic about these and even he recommended them; so I guess you can consider me "sold" 8)

Thanks again for the help everyone. I at least learned some things.