Just bought first boat but has low top speed

[diybrad]

I just bought a 1990 thundercraft 1770 with the 4.3 V6. I had it out with the wife and dog and almost empty tank of gas. I could only get it to 36 MPH on the GPS. The prop is a 21p but i do not know the diameter. I have changed all the fluids and cleaned all the crap off of the flame arrestor but i havent done a compression check yet. I plan on doing that this week. I can only get the engine to about 40-4200 rpms even with playing with the trim. I figure a boat this small should get to aleast 45 with that size engine. What could be the possible issue here?

I am new to boats and want a few areas to start looking. I am guessing just like in a car if the engine is hurt the top speed would be affected. I used to build auto race engines so engines are easy, I just dont know all the dynamics that would determine top speed and performance on a boat.

Thanks for the help.

Brad

 

[Chris1956]

Re: Just bought first boat but has low top speed

Brad, A compression check is in order. Do you trim the motor up when cruising? Trim will help a lot with top speed. Check for extra weight in the boat. Maybe floatation foam is saturated, adding several hundred pounds?

 

[spoilsofwar]

Re: Just bought first boat but has low top speed

WOT RPM for that engine should be 4200-4600... You may just be slightly overpropped or there may be other issues at play like Chris suggested.

 

[diybrad]

Re: Just bought first boat but has low top speed

i have been playing with the trim. when trim is all the way down i am at about 32mph but when i play with it i can get 35-36 then if i got to far i loose speed again.

There are a few soft spots on the deck near/in the engine bay. i planned on pulling the engine and redoing the decking/stringers and the transom if needed at the end of the season and checking for saturated foam. hope i dont have to pull it sooner. I would like to enjoy it for a season first.

I am not familiar with engines with mechanical fuel pumps when doing a compression check. I am an EFI guy. Do i need to pull the mechanical fuel pump off for the compression check or leave it? I know i need to warm it up and pull the leads off of the coil.

Thanks

Brad

 

[jkust]

Re: Just bought first boat but has low top speed

i have been playing with the trim. when trim is all the way down i am at about 32mph but when i play with it i can get 35-36 then if i got to far i loose speed again.

There are a few soft spots on the deck near/in the engine bay. i planned on pulling the engine and redoing the decking/stringers and the transom if needed at the end of the season and checking for saturated foam. hope i dont have to pull it sooner. I would like to enjoy it for a season first.

I am not familiar with engines with mechanical fuel pumps when doing a compression check. I am an EFI guy. Do i need to pull the mechanical fuel pump off for the compression check or leave it? I know i need to warm it up and pull the leads off of the coil.

Thanks

Brad

Sorry to hear about the soft spots. They equal rot and water intrusion and possibly water logged foam adding a lot of weight to the boat. Does it sit very low in the water?

 

[tpenfield]

Re: Just bought first boat but has low top speed

I just bought a 1990 thundercraft 1770 with the 4.3 V6. I had it out with the wife and dog and almost empty tank of gas. I could only get it to 36 MPH on the GPS. The prop is a 21p but i do not know the diameter. I have changed all the fluids and cleaned all the crap off of the flame arrestor but i havent done a compression check yet. I plan on doing that this week. I can only get the engine to about 40-4200 rpms even with playing with the trim. I figure a boat this small should get to aleast 45 with that size engine. What could be the possible issue here?

I am new to boats and want a few areas to start looking. I am guessing just like in a car if the engine is hurt the top speed would be affected. I used to build auto race engines so engines are easy, I just dont know all the dynamics that would determine top speed and performance on a boat.

Thanks for the help.

Brad

How much does the dog weigh ???

The RPM's seems a bit low, but only slightly so. A little less prop will get the RPM's up, but probably about the same speed.

So, I would get a baseline on the engine, compression, etc. Also, make sure the throttle is opening up all the way, in case it is just a linkage adjustemtn, etc.

 

[diybrad]

Re: Just bought first boat but has low top speed

Sorry to hear about the soft spots. They equal rot and water intrusion and possibly water logged foam adding a lot of weight to the boat. Does it sit very low in the water?

I knew it had the soft spots before i bought it. It doest appear to sit to low. I will get a picture of it this weekend while sitting in the water with no one in it.

 

[hostage]

Re: Just bought first boat but has low top speed

I am betting it has a lot of water logged foam in it.

 

[oldjeep]

Re: Just bought first boat but has low top speed

Might be worth driving it to the scale

 

[diybrad]

Re: Just bought first boat but has low top speed

I am betting it has a lot of water logged foam in it.

I am guessing the only way to fix that problem is to tear up the floor and remove old foam and then new foam and wood decking?

 

[jkust]

Re: Just bought first boat but has low top speed

I am guessing the only way to fix that problem is to tear up the floor and remove old foam and then new foam and wood decking?

Yes decking, stringers, foam, etc. This situation has been repeated countless times on older boats. It is an involved job requiring multiple skillsets and a good bit of time but the restoration section here walks you through it.

 

[BonairII]

Re: Just bought first boat but has low top speed

I am guessing the only way to fix that problem is to tear up the floor and remove old foam and then new foam and wood decking?

Only way to correctly repair it is to completely gut the boat and completely rebuild the inside of the hull(stringers, transom, and foam).

It's a huge undertaking. Lots of work and lots of money.

 

[diybrad]

Re: Just bought first boat but has low top speed

How much does the dog weigh ???

The RPM's seems a bit low, but only slightly so. A little less prop will get the RPM's up, but probably about the same speed.

So, I would get a baseline on the engine, compression, etc. Also, make sure the throttle is opening up all the way, in case it is just a linkage adjustemtn, etc.

If i remember seeing the specs right it should be about 24-2600 lbs. I may take it to a weight station and get some baseline numbers to check for water logging. I have one right down the street from me.

I have already checked the carb and the carb blades are opening all the way. That was the first thing i ruled out. The prop also has a few good dings on it as well and i am sure that doesnt help any. if i can get all the plugs out tonight i will do a compression test. The plugs dont look really easy to get to.

Thanks
Brad

 

[diybrad]

Re: Just bought first boat but has low top speed

Only way to correctly repair it is to completely gut the boat and completely rebuild the inside of the hull(stringers, transom, and foam).

It's a huge undertaking. Lots of work and lots of money.

i completely gutted and remodeled my 2200 sqft house. I can imagine it is harder than that. Do you think it can be done correctly for a grand or less?

 

[diybrad]

Re: Just bought first boat but has low top speed

should say i cant imagine it would be harder than that but it wont let me edit my post.

 

[hostage]

Re: Just bought first boat but has low top speed

There are a lot of people on this forum that have been there and done that, from what I have read it takes a bit of time and money and you will never get that money back. I hate to say, though we normally hope people come to us before they buy a boat. We would normally advise someone to run, not walk away from a project, unless they really wanted something to do and if they got it cheap. Out of curiosity, how much did you spend on the boat?

I would be a bit more optomistic if there weren't soft spots on you deck, though the symptoms aren't looking so great. Sluggish boat sitting deep in the water that has soft spots. From what I have read the softspots are normally from stringers rotting and the rot goes into the deck and that is helped by foarm holding a lot of water. Not to mention that your tansom could be rotted as well. Some of the guys might suggest you drill some hulls and check for wet foam. Though I would go directly to them as I just browsed their projects. In the end it might be best to call it a loss and come away with a learning experience as many have done.

 

[GOTTAFISH1701]

Re: Just bought first boat but has low top speed

I just bought a 1990 thundercraft 1770 with the 4.3 V6. I had it out with the wife and dog and almost empty tank of gas. I could only get it to 36 MPH on the GPS. The prop is a 21p but i do not know the diameter. I have changed all the fluids and cleaned all the crap off of the flame arrestor but i havent done a compression check yet. I plan on doing that this week. I can only get the engine to about 40-4200 rpms even with playing with the trim. I figure a boat this small should get to aleast 45 with that size engine. What could be the possible issue here?

I am new to boats and want a few areas to start looking. I am guessing just like in a car if the engine is hurt the top speed would be affected. I used to build auto race engines so engines are easy, I just dont know all the dynamics that would determine top speed and performance on a boat.

Thanks for the help.

Brad

You might make sure your bilge pump is working properly.

 

[diybrad]

Re: Just bought first boat but has low top speed

I paid 2400. I love new projects and I am not worried about spending some to have a great running boat with no payment. From what I can tell the transom is it good shape. No hollow sounds and every think looks to be holding in side of the bay.

I just did a compression test and to my surprise the results are below. This was a test with the temperature at 150-160 ish.

1. 163
2. 165
3. 160
4. 165
5. 160
6. 160

I was shocked. All plugs looked okay and the cap and rotor where decent. I am still going to change all the wires plugs cap and rotor.

Other than water logged foam what else should I check? I did discover that I have a cracked exhaust manifold. I drips just a little water. I will have that fixed.

 

[diybrad]

Re: Just bought first boat but has low top speed

You might make sure your bilge pump is working properly.

Bilge pump is working great

 

[QC]

Re: Just bought first boat but has low top speed

Bottom condition, prop condition. Does it have a whale tail on it? Ignition timing. Here's a link that may help: http://forums.iboats.com/showthread.php?t=295070

Your situation is both low speed and low WOT RPM which tells me something is amiss, Can you get us your gear ratio? We also need the WOT RPM confirmed, and accurate (tach).