Boat preforms so poorly---Help

[Vince_TT]

Need help!!

I admit that I am stupid to buy something I have little knowledge on it.
I recently purchased a Renken 2688 which is a pretty big and heavy boat (5500 lb dry wright) to me. The previous owner has put a MerCruiser 454 (rebuilt by Jesper 2 years ago, rate 340HP) in it and upgrade to Bravo I outdrive last season. My mechanic pull the engine out, replace the water pump, adjusted the timing and told me the engine looks good and it does sound very strong. Without engage the gear, the engine roars up to 4600 RPM easily. But, in the open water, it could only reach 3000 RPM which barely brought the boat onto plan, and the worst part was, it only reach 16 mph!! So, my mechanic took a look at the prop and called the prop shop, according to the prop shop, the boat should use a Bravo II with 2:1 gear ratio. the best bid I can use for a Bravo I is a 16x16 prop. I ordered a MerCruiser alum prop one on-line the following week and replaced the stainless one (14x17) mounted on the shaft and found there was no lockwasher and thrust hub behind the prop. Anyway, I put new lockwasher, thrust hub and new prop on it, I did get much better response in low speed but the highest RPM was still 3000 and the speed was 18 MPH... I checked the slip calculator on-line which give me a 40% slip! I bought the boat with full tank of fuel which might sit there for a year.. so, in spite of the WOT performance, what is the problem causing the huge slip? Is the Bravo I not a right fit? the original setup was a MerCruiser 5.7 + Alpha I, is that possible the Bravo is longer than a Alpha so the prop going too deep into the water? the Prop shop recommends a $550 prop for the boat, will this help?? I am clueless now.

 

[HT32BSX115]

Re: Boat preforms so poorly---Help

Uh...how did the boat perform when you tried it before buying?


What is your drive ratio now?

Your boat should probably go around 40 mph at 4600 (WOT)

It doesn't matter what RPM your engine does in neutral. (don't wind it up in neutral....that's NEVER good for any engine)

Have you checked the obvious stuff? bottom covered with sea life etc, water in the bottom of the boat, exhaust system plugged, flame arrester clogged, tune up, old fuel (although 1 year old fuel shouldn't be a problem) etc etc?

 

[Vince_TT]

Re: Boat preforms so poorly---Help

Thanks guys!

Bubba- I originally thought it was over prop too. I went to some on-line prop shops and filled the info- 7000 lb total weight, 340HP, bravo I, etc. the results were either 15x17 or 14.75x17. So I think 16X16 is ok. But, why the high slip?? 40% is a lot for this type of boat! No, I don't think the mechanic has done com-test. This is my next step.

HT32BSX- When I bought the boat, the engine was pull out by a marina in Maryland to fix water pump (VERY POOR JOB THAT CAUSED A HUGE WATER LEAK IN THE WATER HOSE AND EXHAUST PIPE AFTERWARD!!). My mechanic re-did the exhaust and water pump, and also told me the fire arrester was not installed properly and re-installed the fire arrester. So, I assumed these two parts are fine. The fuel might actually sit even longer because the prior owner only use the boat 2 or 3 times last season. Of course, like most of the time I maybe wrong. Should these affect only the WOT but not the prop slip at lower RPM? BTW, my mechanic told me the gear ration is 1.5:1. How can I reduce the slip??

 

[billbayliner]

Re: Boat preforms so poorly---Help

Do not worry about anything but a compression check right now to make sure power plant is ok.

Just peeing in the wind checking other things if the engine is shot. Once engine is verified, then we can move on to next possible issue.

 

[QC]

Re: Boat preforms so poorly---Help

Vince still need to know the condition of the bottom . . . Tell us more about the missing thrust "hub" too. Do you mean the thrust washer between the prop and the gear case? That can't be good . . .

Also, I am just curious here, but why does the engine have to come out to change the water pump?

I don't necessarily agree that we need to look at the engine yet. 40% slip tells me the engine is working, and very hard . . . Are you absolutely 100% positive this is a 1.5:1 ratio?

 

[Vince_TT]

Re: Boat preforms so poorly---Help

The bottom has fresh bottom paint. the boat was kept on a trailer for the whole last season but now is in a slip. the 1.5:1 gear ration is according my mechanic, I can't really check on that since it is in the water now. When I replaced the prop and I did not find the thrust hub neither the lockwasher between the prop and lower unit. Did this damage the shaft of the gearbox?? I am worried.

 

[QC]

Re: Boat preforms so poorly---Help

The lockwasher should be on the end of the shaft, the thrust washer goes on the base of the shaft and the prop rides on that. 100% of the engine's power is transmitted onto the thrust washer. Yes, it could do some major damage I just don't know what actually . . . Seems like it would eat up the end of the gearcase and make some major racket. I have seen stainless props with cast in thrust washers before . . .

Sumpin' is screwy here . . . I am struggling with the 40% slip which I verified. Is she planing? I guess you could see that 40% if there wasn't enough power to plane her. Do you have trim tabs? Are you using them? Down to get on plane and then gradually up some?

 

[JustJason]

Re: Boat preforms so poorly---Help

You should always do a comression test when something is amiss.....
If your compression comes back fine.
your boat needs a tune up to start....
start at the fuel filter and end at the spark plugs.

The funny thing with reman's is.... the installer usually puts on the old carb, the old distributor, and if trying to cut costs... the old cap rotor and wires too. Reman is just the block and maybe heads.

Do not try to band aid the problem(s) by re-propping it....

 

[QC]

Re: Boat preforms so poorly---Help

OK, I did some data searching and 40% slip is very plausible right at that point; inefficient plane or just starting to try to. This could be pitch also. 1.5:1 is pretty steep for a big crusier like that. I am thinking 14" pitch actually and I am not real sure that 40 MPH is in the cards . . .

 

[Vince_TT]

Re: Boat preforms so poorly---Help

Thanks for everybody's comments. I sure will do a compression test soon. I am checking if I can DIY, but every instruction I read on-line says I have to pull the spark plug out one at a time, insert the instrument and then crank the engine for at least 5 turns. so, 8 cylinders means 40 turns. right? is there a tool without the need to start the engine?
QC- I thought the boat was on the plan. I have tried different trim tab angles but the difference was not obvious. I have read from another post saying that if the boat is on the plan, people should see the anti-ventilation plate above the water. Is this correct? I download the lower unit diagram from MerCuriserparts.com, for a Bravo I, the order is lockwasher->thrust hub->prop (from the shaft base to the end), but for a Alpha 1, the order is thrust hub-> lockwasher-> prop. why is the difference? the stainless prop I pull off (Mich gen wheel 14.25 X17) does have a cooper insert with a rubber pad at the end. Does it mean it have build-in thrust washer?
Sorry for some many questions. I have learned a lot just in a day!! this forum is awesome.

 

[QC]

Re: Boat preforms so poorly---Help

Here is a Bravo 1 prop installatrion drawing from the Merc Manual. Alpha props are 100% identical. If you've got no "thrust hub" (more often called a thrust washer) you've got issues unless it is integral to the prop. Item b.:

 

[tmh]

Re: Boat preforms so poorly---Help

just for what it's worth......sorry to kick you when you are down, but why on earth would you buy a boat without a test ride? ......especially if that boat were already in a slip!?!?!?

You need to start finding out what's wrong, maybe hire a good mechanic to start with. I still haven't seen your answer as to WHY the engine was pulled? Did he pull it for performance reasons? If so, he must have checked compression, right??? If not, what was he doing other than running up a big bill?

Search these forums for "underperforming" and the like, then DO WHAT IS RECOMMENDED. It sounds like you know about nothing (not a rip on you, I'm not far off, just a few years of costly learning) so maybe you can't DIY for much of this.

If you are trying to get it fixed cheaply, you are in the wrong activity! You did a very poor job buying, now you will pay for that mistake. Sorry, but that's how it works - I wish it were otherwise.

How old a boat is it, btw.

 

[Vince_TT]

Re: Boat preforms so poorly---Help

TMH- I did learn a costly lesson already..The reason for pulling the engine out was the prior owner hired a marina in Maryland this May to perform the tune up before delivering the boat. I was thinking that since there is a marina involved, this boat should have no major problem...My second mistake! His mechanic found the water pump sometimes did not work and need to be replaced. As I mentioned, the prior owner droped a 454 into the engine compartment which was designed for a 350, therefore the only way to reach the water pump is to pull the whole engine out. Back then I was traveling overseas and finally came back around Independance Day; I called the marina in Maryland and someone told me everything was checked and run prefectly; the boat was sitting on a trailer ready to go. I hired a mover to bring the boat back to Jersey in the middle of July. Right after launching the boat, the transom started taking water. So, I asked my mechanic to check and was told the exhaust pipe connecting the outdrive and the water pump were not installed correctly. I was so pissed and called the marina in Maryland, it turn-out that mechanic never do the water testing!!!But since I was not the person who hired them to do the tune-up, they basically just ignored me. So, I pay thousands to get the water leak fixed-- pulled the engine again. Sad story... BTW, the boat is a 1992.

QC- Please find the attached pic I found on-line, #12 is the toothed lockwasher. Is there different type of Bravo I? Puzzled.....

 

[QC]

Re: Boat preforms so poorly---Help

Yeah, I don't see that view in my Service Manual which is factory Merc. I also don't know where it would go in front of the thrust hub (washer). Yes there are different types of Bravos, and there are also different types of Propellers, but that lockwasher is odd . . . I checked your link and it shows the same prop shaft and bearing carrier part numbers for each style, so I don't know why that lockwasher would be used. No matter what, the thrust hub goes in front of the propeller . . .

 

[willis3368]

Re: Boat preforms so poorly---Help

The lockwasher should be on the end of the shaft, the thrust washer goes on the base of the shaft and the prop rides on that. 100% of the engine's power is transmitted onto the thrust washer. Yes, it could do some major damage I just don't know what actually . . . Seems like it would eat up the end of the gearcase and make some major racket. I have seen stainless props with cast in thrust washers before . . .

Sumpin' is screwy here . . . I am struggling with the 40% slip which I verified. Is she planing? I guess you could see that 40% if there wasn't enough power to plane her. Do you have trim tabs? Are you using them? Down to get on plane and then gradually up some?

I had a old Suzuki motor that one of my techs left off it will eat right in to the case. major damage!!!

It is very noticable you can even tell with the boat in the water, just trim the drive up and look at the case near the prop it should still have paint.

 

[Vince_TT]

Re: Boat preforms so poorly---Help

Willis-- Thanks for the advice. I will difinitely check for the damage next time.

QC- I have done more researches and I did found another drawing similar to yours- without a lockwasher for installing the prop. The title of the drawing is "Bravo I gearcase- standard". Now I am worried, I have put the lockwasher in as show in the first picture. Will it damage the shaft or the thrust washer? More confused.

 

[jaxnjil]

Re: Boat preforms so poorly---Help

if you could turn the prop by hand after you put it its most likely ok
my boat weights 7000 on the trailer and ready to run.
my alpha I can swing a 15x21 mirage II at 3600 ft elevation with a 1.65:1 gear. top gps speed is 43 mph at 4300 rpm
the 7.4 alpha's came stock with a 1.32:1 gear so i wouldnt think a 1.50:1 would be a problem.
i dont trust the tack on my boat as i have checked it with a photo tack and its never been right and has been from 100 to 600 rpm off and any where in between.
i cant tell you what your problem is but you mignt want to check the tack before you trust it and posted my info for you to have something to compare to

 

[45Auto]

Re: Boat preforms so poorly---Help

Assuming the boat comes on plane somewhere in the low 20's, 40% slip at 16 MPH is about right. As the boat speeds up the prop slips less, requiring more power to keep accelerating till the boat planes, at which point the power requirement drops significantly. When the boat hits plane you'll probably be somewhere in the 30% slip range. At max speed it'll be around 12% - 15%.

My guess is that something is wrong with the engine and it is running out of power at 16 MPH and 3000 RPM to go any faster. It can't push the boat any faster as the prop attempts to hook up more. Revving up in neutral means nothing, takes no power to rev the engine up with no load on it.

I would be looking into compression, carb linkage, timing, firing order, etc. Sounds like some of the mechanical work on the engine might have been less than the best .........

 

[tmh]

Re: Boat preforms so poorly---Help

yeah.....sounds like you don't need any lectures right now....sorry.

I wonder why the previous owner put the larger engine into it? How long did they run it w/ the new engine??? I wonder if the boat could be waterlogged? Have you weighed it?

 

[Vince_TT]

Re: Boat preforms so poorly---Help

Lectures are good..:redface: otherwise I can't learn lessons.

Jaxnjil- you are right, I won't trust 100% on my tach's reading neither. I bought a photo tach as well and will varify the really rmp next time.

45auto- after all of these discussions, I believe there is something wrong with the engine. I noticed that neither RMP nor speed would change from the 3/4 throttle to WTO, I orignially thought this was caused by wrong prop and installation. But now everything points to a engine problem...

TMH-- the prior owner just wanted more speed. Unfortunity it did not work out for me. I did clean up all the fish boxes, live wells last week and at one point the boat reached 20 MPH. I can't wait to get back working on compression test, checking the carb, etc, but the weather.com said it gonna to rain hard on Saturday. Man, can we learn how to drive rain away like China does for Olympic?