one of my friends in my flight school class bought a 17' inboard/outboard. had a 4cyl and a kinda puny looking stern drive with no trim. I think it was around 120-140 hp. The transom was rotting, so he had to pull the drivetrain out to repair it, and wanted to drop a bigger engine in...some sort of V6.

I told him that the cheapest thing to put in there would be a 305 or 350 chevy...so the idea of more power/less money convinced him pretty quickly.

He has the engine and drive taken off, and he's cut the rotten portion out of the transom (pretty much the middle third on the top, and about the middle 2/3 towards the bottom. He has 2 sheets of plywood clamped and laminated to use as the "patch"...probably a total of about 2" thick.

He has a local boat shop trying to source him a stern drive (with trim) to mate up to a SBC, and he priced the Autozone/Advance carbed 350 longblocks.

I went to his house tonight to do some flightplans, and while i was there, i lent my engineer's eye to the boat and helped him figure out how to brace things up. We don't want to crack the new transom or the hull.

Should be a pretty interesting project...I'll try to get a series of pictures of the progression going...I told him I'd help, and there's another guy in the class who used to build engines for a Pro Stock drag car team or something, and he said he'd help.

2/1/2006 1:19:14 AM

You are probably doubling the weight of the back of the boat, which will severly decrease handling. Depending on the hull shape, it may not plane anymore, and might just create a big wave making dog of a boat. I would look around, if you can find that hull shape with a big motor, it might be worth considering. Otherwise, what are describing is probably not worth it.

2/1/2006 6:53:44 AM

damn Ben, you think? that's not what I want to hear.

You're the boat guy, but I figured that doubling the power would more than offset adding a couple hundred pounds to the ass end in terms of getting it planing.

2/1/2006 8:15:21 AM

dont car motors and marine motors turn opposite ways?

2/1/2006 8:19:38 AM

maybe some of them, i don't know...hell, some car motors turn opposite ways!

but lots (most?) inboard/outboards run automotive engines. If I had a dollar for every ski boat with either a 350 chev or 351W...

2/1/2006 8:22:52 AM

Boats are a bit different than cars, the hull shape and relation to the waterline plays a big role in how fast they go, handling, etc. Also, the forces are sometimes distributed oddly, so that even if you beef up the transom, the added HP could cause the boat to fall apart.

I once saw a 41 foot sailboat with a 20 HP diesel, it would cruise 5-6 knots I think. The owner replaced the diesel with a turbo charged chevy v-8. It would then cruise at 6 knots (max on flat water) but it threw up a hell of a wake.

Study on it a bit before you go ahead with this.

2/1/2006 10:03:34 AM

^ aren't displacement hulls limited by their length?

2/1/2006 11:06:22 AM

I think it will still plane out. My boat planes out fine with three people on the back seat and that's got to be close to 500 lbs back there. Granted, it's 4' longer, but still.

And yeah, hulls are designed to go a certain speed and it takes a minimal amount of power to get to that speed. Doubling the horsepower might only pick up 5-10 MPH in top speed, but it'll get there a whole lot quicker (while guzzling gas at an astronomical rate.)

[Edited on February 1, 2006 at 11:30 AM. Reason : s]

2/1/2006 11:21:01 AM

an aluminum v-6 would be a better choice for that small of a boat, or a 4.3, those are cheap and easy to find, and will mate up to a chevrolet drive, biggest issue is the fuel tank, if it has a rear or center belly tank, you may have some mojor clearance isseues, also you need a marine carb and fuel system, am automotive one would be flat out dangerous

2/1/2006 12:50:02 PM

lexus/toyota tundra aluminum dohc v8

2/1/2006 1:28:14 PM

and a computer and a custom machined drive for that, that'd be a great option for about $10,000

2/1/2006 1:45:45 PM

I dunno, I saw a completed boat with a blown lexus v8 and open headers. It looked pretty fast, sounded nasty

2/1/2006 2:50:55 PM

though based on auto blocks, marine motors can be quite a different animal. you can't simply toss a car motor in a boat.

i'm assuming this boat probably came with the gm 2.5 "iron duke" motor, which isn't any lightweight. i highly doubt the 350 would adversly affect the handling or planing, like Shack said boats are designed to handle at least a few extra lbs. in the stern from passengers.

what's his intended use for it? really may be better off just sticking w/ the 4 cylinder.

2/1/2006 3:04:34 PM

Quote :

"^ aren't displacement hulls limited by their length?"

The generally accepted principle of the maximum theoretical hull speed of a displacement vessel is determined by the square root of the waterline length multiplied by 1.34

There's something that can happen to sailboats (powerboats usually fall apart before it happens to them, and most powerboats are planing hull designs) when they exceed their maximum hullspeed.

If a displacement hull begins to exceed is theoretical hull speed, the hull is litterally sucked down under the water as the boat accelerates. It may be hard to notice at times, but then when the water is only 6" below the deck, it becomes evident quick.

Its really weird when it happens, I've only been on one boat that it did it on, and we backed down immediately. I think we had over 5000 lbs of pressure on the back stay hydraulic ram at the time just to keep the stick up.

As for your power boat, take a gander at this:
http://www.yanmarhelp.com/o_perf.htm

2/1/2006 6:21:35 PM

i think any planing boat can pick up more speed until you pull the hull out of the water. i mean i've worked on a 23' stingray (back when the transom deck wasn't measured in the overall length) but he pick up a ton of speed when we changed the cam and injected it (new prop too) and then a ton more when we called procharger. in a farily standard looking boat it pushes 100 (96 gps).

But a displacement hull like some sailboats and most ships... they're not designed to come on plane and will just push more water if you try and push them harder.

Quote :

"The reason that everybody hasn't rushed out and bought one is that displacement hulls are slow! Their design limits their top speed to something called "hull speed," and they can exceed it but little, and with great diminution of many of their benefits (see above). Hull speed is calculated in knots (a knot being a little more than a mile per hour) using: 1. the square root of the water-line length, and 2. the constant 1.34 for length in feet or 2.43 in metres. So: hull speed = K * SQR(water-line length) So, for a 30 footer with a 25 foot water-line length, we take the square root of 25, which is 5, multiply by 1.34 to get 6.7 knots. Or, a 12-metre boat with a water-line length of 9 metres works out to about 7.3 knots. (Incidentally, 12 metres is about 39 feet, so notice that you don't gain much speed as you get longer - that little square root thing is holding us back!) "

2/1/2006 6:30:21 PM

What if it has the Mercruiser 3.7 four-banger? That's definitely an interesting engine...and a little on the heavy side as well. I think it uses one Ford 460 head...either that or a custom casting.

What's wrong with sticking with the original stern drive? Evidently the fella has money to dump, or he got the boat for a ridiculously cheap price to want to yank out the whole propulsion system and throw in another.

I'd have just bought a boat with what I want in it already.

2/1/2006 6:53:28 PM

^could be, i just noticed the hp he posted. the gm 4 banger was rated around 100-110 i believe.

2/1/2006 7:01:38 PM

the iron duke isnt all that heavy. at least the one in my truck didnt the 5 times i had it out. i think the marine versions were 3.0l strokers too.

it could be a volvo penta too.

thats all i know about boats.

2/1/2006 7:06:27 PM

Quote :

"What's wrong with sticking with the original stern drive? Evidently the fella has money to dump, or he got the boat for a ridiculously cheap price to want to yank out the whole propulsion system and throw in another."

He said he thinks that his old engine is a Volvo.

He was originally gonna use his old stern drive, but decided that he's rather have one with trim and that he wouldn't have to try to figure a way to adapt to a SBC. I mean, maybe Advance Adapters could hook him up, but he just figured that it would be easier to snag the right stern drive for the job (which he wouldn't have to worry about breaking, either). plus, the final drive ratio on his old drive was pretty short. tough to get enough prop on it to put twice as much hp to use.

2/1/2006 7:31:56 PM

^^Joel, the engine you had was actually a Pontiac Super Duty four, not a real Iron Duke.

The Pontiac engine is a 151 and has a crossflow head.

The original Iron Duke (and the versions that Mercury uses, both 2.5L and 3.0L) is a Chevy engine, and displaces 153 cubic inches (at least the 2.5; basically half of a 307).

Oh, and if it's a Volvo Penta stern drive, then the engine is a B20 or B21...both pushrod engines, both used in Volvo Penta stern drives...no others are used.

All other manufacturers of stern drives use either the Chevy II (Iron Duke), Pontiac Super Duty, or Ford 2300 (OMC) fours.

[Edited on February 1, 2006 at 7:53 PM. Reason : more useless info.]

2/1/2006 7:50:11 PM

^gg, for clearing "the duke" (no, not you josh lol) up.

fyi, simple rule (but some exceptions) rule of thumb for outdrives: volvo penta= gm motor and mercruiser= ford motor.

kind of odd considering in the automotive world ford now owns part of volvo, but i digress....

2/1/2006 11:49:37 PM

Quote :

"volvo penta= gm motor and mercruiser= ford motor."

do what!!?!?! mercruiser has been using 305s 350s 454s and 502s ever since i was born i think at least as long as i can remember.

Omc was the only one who i've run into ford with and they had arguably the worst outdrive ever made. ford also used to be THE motor of choice for ski boats (351w)

unless you're talking 4popper specific then i wouldn't know cause i don't have any experience with 4cy i/os

[Edited on February 2, 2006 at 3:14 AM. Reason : .]

2/2/2006 3:12:03 AM

buy a skyline engine w/ twin turbos.

2/2/2006 6:14:45 AM

i thought OMC was GM or something?

the boat im working on has an OMC 3.0l Cobra 4 banger. from what ive read its supposed to do 130ish hp. or maybe it just had a GM carb cuz the carb rebuilt kit i got for it was GM i think.

oh i dont know wtf i'm talking about.. this is my dad's project. i just hope the bitches goes someday

[Edited on February 2, 2006 at 8:54 AM. Reason : adfdf]

2/2/2006 8:53:59 AM

OMC also uses GM engines. They did a stint in the '80's where they used Ford engines religiously.

2/2/2006 10:41:53 AM

Quote :

"unless you're talking 4popper specific"

yes. alot of the time for v6's also (4.3 and 3.0).

^^OMC was a huge group that had a large number of brands under it's belt, and rebadged alot of parts with their name. they were a real powerhouse in the boating industry through the 80's/early 90's. they never produced anything extremely amazing, but they had the market on lockdown for solid and generally affordable boats/engines/parts.

they went into a downhill spiral in the late 90's or so, and were broken up around the turn of the century. i think genmar and bombardier bought up the majority of the brands iirc. i don't really know what their downfall was, i know they were dumping a ton of money into developing a new drive system at one point. i think there was an upper level management change that made poor decisions also.

2/2/2006 1:34:50 PM

Yeah, I think OMC was completely bought out by Bombardier. This includes Evinrude and Johnson...I don't think any stern drive stuff exists anymore.

2/2/2006 7:24:38 PM

haha, you should try finding parts for my 85 bayliner with a force 125, took me 2 years to find someone in washington with oe trim/tilt pump parts. and had to pull the power head to replace the normally easy colapsable exhast with a piece of stiff universal marine hose, had to get dad to custom machine carb linkage bushings, and had to put accel motorcyle coils on there when two crapped out.

2/2/2006 8:48:03 PM

i wanna boat

2/2/2006 8:54:03 PM

Oh yer sumfoo1, alright...

You gotta be a fucktard to buy a boat to begin with.

2/2/2006 8:58:42 PM

i know they're just 1 big money pit but i <3 the water.

2/2/2006 8:59:34 PM

build your own, its always a safe idea, just need some plywood and duct tape

2/2/2006 9:12:14 PM

just picked up my new money pit on wednesday, more pics in gallery!

2/2/2006 10:43:03 PM

ok, over here at Adam's house right now.

It's a Glastron boat, and the engine/drive is a B20/Penta.



Question: does a 4.3 GM V6 share a small block's bellhousing?

2/2/2006 11:16:33 PM

^^chatlee? good group of people down there.
^oh that thing is older then if it's got a volvo b20. yep, 4.3 shares the same 90 degree bellhousing as small blocks.

2/2/2006 11:22:40 PM

^ yes, my bro-in-law works down there in the shop, and the wife's family is friends with the owners, so a natural fit, it's a 2006 stingray 195lr with the merc. 4.3 carb with Alpha 1 drive. haven't had it in the water outside of their test pond, can't wait for the weather to break!

2/2/2006 11:41:20 PM

cool, myself and my family have done quite a bit of business there over the years. hell, tomorrow is looking like a perfect day for the lake (as perfect as a winter day ever is).

2/3/2006 12:02:14 AM

i'm considering putting in tommorow after a quick trip up to overtons to pick up some last minute items, we've (family) done quite alot of business with them and with us getting into it i'm sure this won't be our last boat from them. in the future probably looking to go up to a 23 or 24 foot sport/cruiser or possibly a center console if the urge strikes. too bad they can't get sea ray because i have a feeling that's where we'll eventually end up in a bunch of years in a 26-30' sundancer or something of the like

2/3/2006 12:08:00 AM

Maybe so, but Stingray is where it's at as far as hull design and efficiency.

2/3/2006 7:05:02 AM

Norco isn't too shabby either they're getting more speed out of thier hull w/ smaller motors than the rest of the crowd... like 7-10 mph faster than baja w/ the same motor.

(damn i wish baja would just go make a f'in stepped hull) i mean they're cheap and made by searay who imho has some good quality stuff.

[Edited on February 3, 2006 at 7:30 AM. Reason : .]

2/3/2006 7:23:41 AM

yeah i have done a bunch of research and have been waiting for about 2 years to buy this boat, and I am very happy with the purchase so far. I wish Stingray made something bigger than 25 though for that later in life boat! oh well one can dream!

2/3/2006 9:09:37 AM

check out nordic... seriously

sorry i brain farted earlier... its nordic

2/3/2006 11:15:25 AM

Nice Stingray. I wish I wasn't so stuck on getting a wakeboard boat because you can get so much more for your money in a normal I/O runabout.

2/3/2006 11:27:33 AM

As far as hull designs/shapes are concerned, powerboats see a lot less innovation than sailboats. Whereas the tendency to make a powerboat go faster is simply add more HP, with sailboats, you can't just go put a bigger sail on it to make it go faster.... almost every single gain in efficency is made with hull/rig design and weight reduction/placement.

2/3/2006 12:47:20 PM

^^ thanks, yeah i'm not too big in to skiing or wakeboarding, yet.. but just something to take out on the lakes and have a good time on my short weekends or my days off during the week as we're normally at the beach every 2 day weekend i get!

2/3/2006 2:06:35 PM