Result of under-powering a boat? (Half of what it's rated for)

[JBMN]

Being still a little bit of a newbie, I'm still learning...... After having my 1550 Forester Diva for a few years, I want to step it up. Like Chief Brody says, "you're gonna need a bigger boat." So I have my eye on an 18' Sea Ray - BUT- it needs a motor and controls. I absolutely love my 70hp Yamaha on the Forester.......but my Forester is in need of some serious vinyl restore. So I'm thinking of pulling the Yamaha and perhaps using it (and needed controls) off the Forester and using them on the Sea Ray....which is rated for 140hp. So I'm under-rated by half. What's the result? (Lower top speed? Less maneuverability? Not getting on plane?) I use the Forester now as a 'cruiser' and dive platform, and have once towed a kid on a tube...... So I don't need a ton of power. Thoughts?
p.s. Not having a motor on the transom of the potential new boat, I'm not 100% sure how best to see if the transom is rock solid. (?)

 

[smokeonthewater]

The result depends on the boat and on your expectations...
It MAY never get on plane or it may do just fine with a lower top speed...
If you tend to load up with lots of people that could also be a problem...

The correct prop would be mandatory.... The only way to know for sure is to try it... There will be some that post up saying it can't be done...

I've seen a few examples... My wellcraft V-20 is rated for 200 hp and they are known to perform fine as long as not heavily loaded with 90 hp.
I had a 26' flying bridge carver and I removed the blown 260 hp v8 and replaced it with 165 hp i6... It would plane very quickly at 2/3 throttle...

 

[bonz_d]

Sure it could be done but I don't think you'll be happy with the results. 1st thing you would have to do is change the prop to the smallest pitch you can find and hope it will get it to the operating range of the engine. If not it will overload the engine and could cause premature stress and wear. It will also limit your load capacity.

 

[WIMUSKY]

Extreme frustration/dissatisfaction from the guy at the helm.....

 

[GA_Boater]

It all depends on your idea of cruising. If you mosey around, the 70 is fine, don't expect much. If you really try to go places. you will be disappointed.

 

[Alumarine]

Is the Yamaha the right shaft length for the Sea Ray?

 

[ondarvr]

As Smoke said, it may work, but you need to try it to find out. I have a 19' center console rated at 150, it came with a 90 on it and worked very well. Put a 50 on it once and it would still get on plane. I now have a 200 on the transom, a 115 would probably be the best motor for my boat.

 

[Frank Acampora]

I had an 18 foot Renken powered with a 140. After I blew the prop shaft and a piston on the 140 I repowered the hull with a Chrysler 75 that I had lying around doing nothing. Took her out cruising on the Chesapeake Bay for a day.

I needed to drop down 2 pitches on the prop (19 to 17). The boat planed nicely with just me in it and topped out at about 25 MPH. Sadly, that simply wasn't enough for me so before using the boat with more people or for water sports, I swapped engines again for one with a lot more power. (125).

Most hulls plane at somewhere around 18 MPH so if the 70 can get "over the hump" you should get at least marginal performance. You might even be satisfied with it.

 

[thumpar]

As long as it will plane out you are fine. It will just be slower. If it doesn't plane out you are just plowing in the water and using lots of fuel. I would think it would plane with that engine though.

 

[JoLin]

For me, the load I carry in the boat would be the determining factor. A pitch change will probably get you on plane and motoring along in the low-mid 20's mph. Add a few people, gear and fuel load and you may well not make it out of the hole. Look, if it's relatively easy for you to do the motor swap and try it, by all means do so. Different hulls react very differently to power and load.

My 26' Carver is 1000 lbs heavier than my last (a 27' Four Winns Vista), but it reacts better with the Merc 4V V-6's than the FW did with Volvo FI's. The Carver has the same beam and more sail area (wind resistance with the flybridge) than the FW, but also a little less hull deadrise. With a light load I can keep the Carver on plane at 17-18 knots where the FW would usually 'fall off' at anything under 20.

 

[bonz_d]

Here is an example that I've used before. I have a 1988 Alumacraft Classic 16 of which the also make a DLX model which is a slight bit heavier. These boats are rated for a max hp of 75. I've run a 60hp 2 cylinder Johnson on it and it does extremely well and believe a 75hp would be over powered. I also see a lot of these same boats running 40hp OMC engines on them and they must do fine or there wouldn't be that many of them around. Oh, and these boats are loaded up with multiple batteries, big trolling motors and large live wells.

 

[skibrain]

How do hull weights compare?
Dead rise?

My Neighbor got a really great-shape 18' searay. Solid, heavy, quality boat. Sorry can't remember the model but it was rated for 140 or 150outboard. Came with a Merc 90 and he said it was a dog. They were never happy with it for water sports, especially if more than 2 in boat. I have no idea how much he had worked on prop optimization - but he really didn't like it.

 

[ondarvr]

The new hull.

One reason people get rid of a hull and no motor is because after investigating fixing the rotten transom and stringers they decide it's not worth it and then dump it on some unsuspecting newbie.

I would make sure to investigate the hull very well before turning over any cash, if any part of it is rotten the price should drop to free, or very close to it. Even one soft spot in the floor is a sign of a total rebuild being needed.

 

[JimS123]

When I was a kid my neighbor had a wooden 18 footer that weighed a ton and it was powered by a 75 HP Merc. That was the biggest HP motor of the day and we thought we were in boating heaven. Today the internet recommendation would be to power that same boat with a 200.

My neighbor today has a Formula with twin 454's and he thinks it not big enough.

This is a silly conversation.

Most ANY HP OB will perform satisfactority on ANY hull, within reason, with the correct prop. Michigan has quite a selection - all you need to do is prop the WOT at max speced engine rpm. Most any boat will be able to plane out properly, though top end speed may not be to your liking.

We are all spoiled today.

The OP's 70 Yami will be just fine if propped right, unless racing everybody on the river is the requirement.

 

[JBMN]

This is why I love this site- lots of good gouge on here! My take-away is that this could be an exercise in frustration. Thanks for the input (forgot to look into the shaft length and prop pitch issues)