Question re: engine/hull potential combo

[CWKboat]

Hi, just wanted to solicit a few thoughts re: the potential union of A 1987 Mercury 115 inline 6 with A 2002 Larson 20 footer?

I recently picked up the Larson in a pretty good deal/winter project. The 115 is in great shape and was just rebuilt. The Larson is a pretty big boat and is rated for a max 175 HP.

I'd be happy with 30 MPH or low 30s on this setup. I'm not in the market to buy a larger engine right now, so was curious as to some opinions re: how that 115 would do on a 20 footer of fairly recent vintage like the Larson (VEC hull/all composite)? Just want to make sure I wouldn't be too underpowered?

Thanks for any opinions or thoughts!

 

[Friscoboater]

Re: Question re: engine/hull potential combo

Personally I would think it would be under powered. There are times you will need that power, and if you do not have enough power, you will actually use more fuel over working the smaller engine.

 

[Fireman431]

Re: Question re: engine/hull potential combo

You might be able to hit 30 @ WOT, but it would fall into the mid 20's at cruising speed.

A 115 is really underpowered to push it to your requested speed. Generally, a 20' will require in the neighborhood of 150HP to cruise easily in the 30's and hit low to mid 40's at WOT. There is a formula used to derive the potential speed of the hull, but I don't know it. I'm sure someone will post it for you or check Google.

That being said, the older engines derived their HP ratings from the propshaft where as newer engines get them from the crankshaft. So the 150HP engine will probably only put out 135HP to the prop where your 115HP probably has 115HP (when new) at the prop already.

One way to find out...put it on and see.

 

[ziggy]

Re: Question re: engine/hull potential combo

the older engines derived their HP ratings from the propshaft where as newer engines get them from the crankshaft.

i thought it was the other way around. old was crankshaft hp rated and new is propshaft rated...

anyone have any documentation as to which way it is? perhaps o/b's are rated differently?

i think the 115 will likely be lame too.

kinda apples to oranges, but my sig. boat is max rated at the 165hp it's got. an i/o. i get 42mph @ top end of wot spec. if i had a 110 in it or the likes. i would bet i wouldn't like my boat near as much for being under powered..

 

[NYBo]

Re: Question re: engine/hull potential combo

i thought it was the other way around. old was crankshaft hp rated and new is propshaft rated...

You are correct. Fireman has it backwards.

Using the 70%-of-maximum-HP rule of thumb equates to 122.5 HP as the minimum for decent performance. YMMV!

 

[kmarine]

Re: Question re: engine/hull potential combo

A bit underpowered perhapse. Are you planing to use this boat for watersking or towing inflatables? You will require more torque. Many manufacturers do put 115 hp outboards on 20 ft boats. My brother in law had one a few years back. The boat would run at about 35 mph, but had poor hole shot and could barely drag skiers out.

 

[Fireman431]

Re: Question re: engine/hull potential combo

You are correct. Fireman has it backwards.

Story of my life. Thanks for the correction. It actually makes more sense that way!

 

[Frank Acampora]

Re: Question re: engine/hull potential combo

While mercs have a reputation for under-rating the horsepower of their engines, I don't think you will be too happy with the tower of power 115. It is rated 115 at the prop as all engines after 1982 are. But, there is only so much that 115 can do. Your 115 will move the boat and plane it. It will be OK for tubing but skiing may be problematical. I agree with Fireman that top speed will probably be about 30 and acceleration will be marginal.

Now, the newer Mercs: I repaired a mid to late 90s oil injected, four cylinder, four carb Merc 100. That thing was a BEAR! It drove a 21 foot round bottom cuddy at 39 MPH with three people in it. I swear: That engine was putting out well more than the claimed 100 HP.

 

[captainjammer]

Re: Question re: engine/hull potential combo

I have a 115hp inline 6. you will need to go to a bigger diameter and less pitch, you will do just fine. Mine is on a 19ft older glass starcraft, 40 mph and still could use some more pitch. You probably wont be cruising at 30 mph, propped correct wide open ill bet it would run 32-34 mph? cruise about 26-28 at 3/4 throttle. awsome motors. and they sure like to drink gas. We had a 115 on a work boat (28 ft steel) it would actualy put it on plane (without welder and tools) in it. It would use a 12 gal can in 20 min. lol. good luck

 

[lprizman]

Re: Question re: engine/hull potential combo

as stated will do the trick,,get you on plane,,,not a world beater,,,but hey,,your on the water

 

[proshadetree]

Re: Question re: engine/hull potential combo

Put it on and get to boating. I saw that same boat with an old 85 Johnson and the guy loved it. Topped out at like 30 and pulled a tube. After all the 175 is max rating.

 

[CWKboat]

Re: Question re: engine/hull potential combo

Thanks for the great and informative responses all. I wouldn't be planning on doing a lot of water skiing or tubing, although the boat does have a ski pole.

I'm leaning toward putting the motor on and trying it out for a season and then down the road trying to upgrade to a 150. I also have a manual jack plate so I should be able to set the 115 up to at least get the most out of it.

Of note for the Merc guys in the thread, I also have a mid 70s inline 6 150 HP that I also considered putting on it. It's rebuilt and runs awesome, but I leaned away from that option as the mounting holes are different and it has the older external trim. It's before the modern HP rating, so I figure it would be 125-130 HP or so, but just felt that any minimal gains would be negated by the older trim and differing bolt pattern.

In short, I suppose I will plan on putting the 115 on while at the same time looking for a good deal on a bigger engine!

 

[NYBo]

Re: Question re: engine/hull potential combo

It is rated 115 at the prop as all engines after 1982 are.

1982? So my '87 and '88 85 HP Force outboards are 85 at the prop?

 

[CWKboat]

Re: Question re: engine/hull potential combo

Well, to update, I just got a good deal on a '87 Mercury Black Max 150 HP this week, so will be going with that engine and selling the 115 HP. I'm pretty happy with the potential combo and will rig it ASAP.

 

[Fireman431]

Re: Question re: engine/hull potential combo

I think you'll be much happier with that combo. Find it's cruising sweet spot (my history with that combo shows approx 4000-4200, depending on prop & trim). You should be able to cruise at 35 MPH and WOT arounD 42-44.

Good job on finding the Black Max.

 

[CWKboat]

Re: Question re: engine/hull potential combo

Just wanted to update this thread: I put the Black Max on the 21 ft 2002 Larson (not a 20 footer as I originally posted). It has a manual jackplate and is currently running a 17P aluminum prop. Still experimenting with motor height, but it's hitting 40 MPH at 5600 MPH so far - maybe can get a little more. Just got it out in the past two weeks after having to replace the lower unit.

Very happy with the combo and glad I didn't underpower it with the 115. A very enjoyable hull and combo.

 

[Fireman431]

Re: Question re: engine/hull potential combo

Going up to a 19P prop will drop your Max RPM's to around 5400 and up your top speed a few MPH. My older Merc was rated at 5200-5500 RPM @ WOT. I pitched up to achieve 5250 @ WOT and got 49 MPH. Play with the prop and trim a bit and tune it out to your liking.

 

[CWKboat]

Re: Question re: engine/hull potential combo

To update again, here's a video taken two weeks ago of the Larson and Black Max combo finally in action. Very pleased with the boat and motor. Thanks for all the replies and help in getting her going:

http://www.flickr.com/photos/49944878@N02/5944277390/in/set-72157627085369005/

 

[lprizman]

Re: Question re: engine/hull potential combo

sounds and looks great!!!!!

Enjoy!!!