Boat not getting on a plane and leaning to side - 1988 Thomspon 240 with 4.3L engine

[Andy82]

Hey guys,

I have a 1988 Thompson 240 Fisherman with a 4.3L OMC inboard/outboard. I had to put some work in to get her running, but have been able to take her out a few times. Still encountering some kinks that need to be worked out, but we're getting there. The times we've taken her out, we noticed that it seems like she's having a really hard time getting up on a plane? It appears that she is just plowing through the water rather than really getting up on it. I was wondering if that is because of the shape of the hull (please see image) or if it is due to a lack of power from the engine?

I suspect it's a lack of power from the engine as I've heard the boat is pretty heavy for the engine. I was wondering if it ma be a good idea to try to add some extra horsepower through some engine modifications? If so, what kind of modifications would yield a moderate HP increase (say 50-100HP?) for the most economical price?

Also, the boat is rolling very heavily to the side when I make turns, much more than I've felt on other boats. This can be a bit unnerving because of the intense list. Any ideas on what may be causing the rolling? I've played with my trim tabs as much as possible to minimize it, and I've definitely made it less severe, but it still feels excessive.

Here's a link to more details on the boat.



So in summary, my questions are:

1. Is it normal that the boat is plowing through the water rather than getting on a plane? If not normal, how can I increase the engine's total HP by 50 to 100HP to help remedy this situation?

2. The boat is leaning very heavily into turns? What might be causing this and how can I remedy it?

 

[roscoe]

Re: Boat not getting on a plane and leaning to side - 1988 Thomspon 240 with 4.3L eng

165 hp motor trying to push a 4400# hull, plus fuel, just not enough power.

I thought all those came with a v8, or twin 4.3's.

Best way to add 100 hp is to put in a 350, and if you are going to that expense, might as well swap the o/c for a mercruiser drive.



As far as the list, ?? Hope it isn't waterlogged. Does it list more in one direction than the other?

 

[steelespike]

Re: Boat not getting on a plane and leaning to side - 1988 Thomspon 240 with 4.3L eng

Is the boat laying down like it is glued to the water?
Does the bow seem to want to steer the boat?
The boat may have a hook.
Lay a 3 or 4 ft straight edge along the keel starting at the transom.
Should be straight no gaps between the bottom and the straight edge.
A healthy 4.3 will plane the boat.What size is the prop?
Numbers should be near the base of the blades or on the ends of the hub.

 

[Watermann]

Re: Boat not getting on a plane and leaning to side - 1988 Thomspon 240 with 4.3L eng

It could be a couple of problems. Weight or under powered with a weak motor. It could be since the boat is 25 years old the flotation foam could be soaked with water adding 100's of pounds to the boat. Take the boat to a scale and see what the weight is now.

 

[Andy82]

Re: Boat not getting on a plane and leaning to side - 1988 Thomspon 240 with 4.3L eng

Thanks for all the feedback so far guys. I think I'll start with weighing the boat and see what we're working with.

roscoe: Ya, agreed, it'd be sweet if I could put in a new motor, but I'm trying to not break the bank.

steelespike: Some good questions there, I'm not 100% on the answers, but I'll give it a shot.

• "Does the bow seem to want to steer the boat?" No, I wouldn't say so. If I'm holding a straight line, there's not really an issue; it's only once I begin to turn that it begins to list. That's natural of course, but the list just feels really excessive.
• "Lay a 3 or 4 ft straight edge along the keel starting at the transom. Should be straight no gaps between the bottom and the straight edge." I'll definitely do this and let you know what I find.
• "A healthy 4.3 will plane the boat. What size is the prop?" Running a 15.5 x 15 prop. Interesting that it should be planing, I'm actually happy to hear that as mostly I've heard the engine is underpowered for the boat. Also interesting about the "healthy engine" part. I may need to do some engine diagnostics, making sure everything is firing right. Whew, that'll be a bit over my head, but I'll just take it step by step.

Watermann and Bubba: You guys are freaking me out, but I have to agree that your hypothesis makes a lot of sense, smart guys. Like I said above, as a first step I'll take her to a scale and see what's up. If the boat does weigh in very heavy and thus we assume she is water logged, what does that imply? Is there any way to dry things out or does it mean ripping out and replacing all kinds of stuff?

I did want to post a follow up to the question that nobody has quite answered yet. Is there an economical way to gain 50-100HP?

Thanks again everyone for the awesome and great feedback so far!

 

[Bondo]

Re: Boat not getting on a plane and leaning to side - 1988 Thomspon 240 with 4.3L eng

I did want to post a follow up to the question that nobody has quite answered yet. Is there an economical way to gain 50-100HP?

Nope,.... Horsepower co$t$ Big Money,... 50 to 100 horse more puts ya into a V8,...

I'm with Steelspike, only I'd use a 6' straight edge against the bottom of the hull,....

If yer hull is Over-weight, that means a complete tear-down to remove the water saturated foam,...

 

[Watermann]

Re: Boat not getting on a plane and leaning to side - 1988 Thomspon 240 with 4.3L eng

To answer your bold question with another question, What model of 4.3 do you have? The reason I ask is I don't know about OMC but MerCruiser has models ranging from 180 - 210 HP in the 4.3L. Believe it or not 30 HP can make a huge difference and be an economical way to up your HP by going to the 4BBL carb if your motor is healthy that is.

 

[southkogs]

Re: Boat not getting on a plane and leaning to side - 1988 Thomspon 240 with 4.3L eng

Andy, THIS thread on power output is yours too - same boat, right?

Have you confirmed you're not waterlogged? What happens when you nose the trailer up and leave the plug out? Any water leakin' out? Even just dripping after a couple hours.

You're leaning, having trouble getting up on plane and concerned that 180HP isn't' enough to put a 24 up on plane. I'm wondering if you're not holding water somewhere.

 

[smokeonthewater]

Re: Boat not getting on a plane and leaning to side - 1988 Thomspon 240 with 4.3L eng

I swapped a 165 hp inline 6 mercruiser into a 5500 lb 26' flybridge cruiser and with the right prop it planed VERY quickly and ran in the high 20's

That style boat WILL lean more than others in turns......


when you say plowing... is that with the bow high or low?

No you can't generally do much to hop up a marine I/O or inboard engine for cheap. If it has a 2 barrel carb, changing to a 4 barrel may help a bit

Do a compression test and check the timing on your engine and also look at the plugs to get an idea of it's current condition

What RPM are you hitting at wot?

Most smart phones are capable of getting a free app that gives you gps mph OR the gps you use in your car works too..... What gps mph are you reaching at WOT?

 

[Starcraft5834]

Re: Boat not getting on a plane and leaning to side - 1988 Thomspon 240 with 4.3L eng

hope it's not waterlogged on one side......if it's leaning too..........assuming your not waterlogged and your engine is providing power around spec.. you said in message u have a 3 blade 15p,, consider a 4 blade 15p... or maybe 12p.... that should help push that boat out of the hole......

 

[crabby captain john]

Re: Boat not getting on a plane and leaning to side - 1988 Thomspon 240 with 4.3L eng

I bought a Thompson 240 Fisherman new in '93. I ran walleye charters on it on Lake Erie for 3 1/2 seasons. I had a 350 and Merc LU. Loved that boat! No lack of power with the 350. Around '94 a guy with more money than sense bought the company. He shortly went under as he knew finance but nothing about boats. 5'ers were barely uncomfortable at low speed. Caught on Lake Erie twice 25 - 30 miles out in solid 8s, very slow going but no white knuckles.

"Also, the boat is rolling very heavily to the side when I make turns, much more than I've felt on other boats. This can be a bit unnerving because of the intense list. Any ideas on what may be causing the rolling? I've played with my trim tabs as much as possible to minimize it, and I've definitely made it less severe, but it still feels excessive." OK-- is she leaning at rest? A fiberglass incased floor was an option, they came with vinyl covered ply. Take the plug out, chock the wheels, put a block (for added height) under the jack and crank it up. Check under the plug hole for 5 - 7 days for water. That big old girl was not made for sharp turns at speed~~ I never had reason to on big water and had no problems at the dock.

 

[rallyart]

Re: Boat not getting on a plane and leaning to side - 1988 Thomspon 240 with 4.3L eng

To answer the question of adding HP, the best way is to repower the boat with a more powerful engine. Unless you are good at rebuilding an engine it is usually cheaper than trying to hotrod what you have. The cheapest way to effectively add 50-100 HP is to add on a Nitrous Oxide kit that you can use to get you out of the hole faster. It provides a chemical supercharging to the engine and is relatively easy to install. The problem is that you need to refill the Nitrous bottle when it gets low. If it's set up right it is probable that you will have no reliability issues especially if you have a 50 HP kit. If it's wrong you might break something.
A narrower hull with a deeper V will list more on a turn. A hull with a bigger outside chine will list less

 

[Bondo]

Re: Boat not getting on a plane and leaning to side - 1988 Thomspon 240 with 4.3L eng

To answer the question of adding HP, the best way is to repower the boat with a more powerful engine. Unless you are good at rebuilding an engine it is usually cheaper than trying to hotrod what you have. The cheapest way to effectively add 50-100 HP is to add on a Nitrous Oxide kit that you can use to get you out of the hole faster. It provides a chemical supercharging to the engine and is relatively easy to install. The problem is that you need to refill the Nitrous bottle when it gets low. If it's set up right it is probable that you will have no reliability issues especially if you have a 50 HP kit. If it's wrong you might break something.
A narrower hull with a deeper V will list more on a turn. A hull with a bigger outside chine will list less

Ayuh,.... Nitrous works at Wot, as in a Drag Race,...

It'll do nothin' but grenade yer motor, usin' it to come outa the hole,...

Which is Why Nitrous isn't used in Boats,...

As I said before, the only way to add 100 horsepower to a V6, is to drop in a V8,....

 

[crabby captain john]

Re: Boat not getting on a plane and leaning to side - 1988 Thomspon 240 with 4.3L eng

Is she leaning at rest or only under power?

 

[tmitchell5188]

Re: Boat not getting on a plane and leaning to side - 1988 Thomspon 240 with 4.3L eng

I had a 1992 model of this boat. I had the 4.3L merc I/o. The boat was a dog. great on gas, but could not get out of it's own way. I was looking at putting in a V8, but I would also have to change I/O gear ratio. I decided to put the money into a newer boat that upgrade the old one. It was a great boat, had very high sides so didn't have any fear of anyone falling over on me. I sold mine in 2004, and it needed the deck (floor replaced). They surveyor said that he thought some of the stringers were also bad. I had the vinyl flooring/plywood deck. The fiberglass floor would of been better.

 

[crabby captain john]

Re: Boat not getting on a plane and leaning to side - 1988 Thomspon 240 with 4.3L eng

I had a 1992 model of this boat. I had the 4.3L merc I/o. The boat was a dog. great on gas, but could not get out of it's own way. I was looking at putting in a V8, but I would also have to change I/O gear ratio. I decided to put the money into a newer boat that upgrade the old one. It was a great boat, had very high sides so didn't have any fear of anyone falling over on me. I sold mine in 2004, and it needed the deck (floor replaced). They surveyor said that he thought some of the stringers were also bad. I had the vinyl flooring/plywood deck. The fiberglass floor would of been better.

I bought a '93 new with the 350 merc and 'glass floor. The only problem I had getting on plane was Lake Erie rarely permitted speeds that high. Topped out around 45 mph and was not bad on gas. Ran charters and you are correct- the only way someone was going over the gunnel was on purpose. Cuddy was just the right size for an overnight. Sold mine in '98 for about what I paid for her. My 1st mate died unexpectedly and that took the desire to charter away. She was built like a tank so I'm surprised you found rot after only 12 years.

 

[greenbush future]

Re: Boat not getting on a plane and leaning to side - 1988 Thomspon 240 with 4.3L eng

Need to check to obvious stuff mentioned and the easy stuff you can check to help you decide includes:

Checking for water under the floor, in the foam and your transom and stringers( you need to drill into the stringers and into the transom to know if the wood is wet/rotted or dry and sturdy). If it's wet you will have some project work to get her fixed and weighing much less.

Check your engine to ensure you don't have a worn out engine. Compression check should give you data to help decide.

Replacing an engine is costly, and if your structure under the floor is rotted (not uncommon) you may then have the ability to make the right choices.
Don't get caught up in just throwing $$ at her until you have the answers and cost's involved in getting her where you want her. You could get lucky and it could be a simple fix, but that's quite rare in the boating world.