Two motors running at same time cause trouble?

[seedpws]

I have a 33hp 1966 motor that I have on my 17' boat. It doesn't go fast enough to slam into the waves. If I start up my 7.5hp kicker it really makes a difference and gets the boat moving. Will it hurt either motor having them running together? They both pump water while moving.

 

[F_R]

Re: Two motors running at same time cause trouble?

It may hurt the 7.5, yes. It may be over revving, if the boat is going faster than a 7.5 normally goes. You would have to check it with a tachometer to find out.

 

[seedpws]

Re: Two motors running at same time cause trouble?

It sounds like it is revving higher than it does by itself. Is that because the water moving allows it to go faster or why would it do this? What will happen to the smaller motor? The 7.5 HP motor has been really good to me and hasn't given me any problems and love it as a backup. It is a 75 merc

 

[iwombat]

Re: Two motors running at same time cause trouble?

You're essentially taking a substantial portion of the load off the kicker by running it in tandem with the bigger motor. I doubt it's healthy for the kicker long-term. And, probably not the safest set-up to run either. You should probably consider repowering your boat and fix it right.

 

[F_R]

Re: Two motors running at same time cause trouble?

Using the theory of screw propulsion, the propeller will "screw" itself through the water at a certain speed at a certain RPM. This is only a theory and is not practical in real life. But it helps to understand that if the boat (and water) is going faster than the rated rpm of the propeller, then the propeller will rotate faster to keep up with the boat. It really doesn't work that way, it works like a pump. But hope this helps.

 

[tashasdaddy]

Re: Two motors running at same time cause trouble?

if it adds that much power, i'd be checking compression on the 33, to see if it is up to par. also a 33 on a 17ft boat is asking a lot, even if it's a stripped fishing boat.

 

[seedpws]

Re: Two motors running at same time cause trouble?

The boat is a 17' 1964 Lonestar aluminum boat. It came with a 1964 75hp that really overpowered it. Went 40mph easy with 2 people onboard.

I keep forgetting my GPS to tell how fast it is pushing the boat. The 7.5HP really does seem to add alot.

I need to check compression and thinking about doing coils points ect cause it sounds like it drops a cylinder once in awhile while under away for like 30 seconds then normal again so not too sure. Alsmo still haven't figured out why I have to have both high and low speed jet adjustments all the way in for it to get to plane the boat...

 

[Texasmark]

Re: Two motors running at same time cause trouble?

Brings back teenage memories of a boat I saw with a 15 and 25 rude on the transom.

The 25 pushed the boat and it kicked the 15 up and out of the water. Apparently the prop/rpm ratio wasn't setup on the engines and the 15 was a hindrance rather than an asset.

Your 7.5 must be matched out pretty good or locked down so that it can't kick up. With that said, If not locked down, then you have it matched and shouldn't be hurting anything.

I think your saving grace is that the 33 isn't pushing the boat all that fast so the 7.5 doesn't have a problem keeping up.

Besides you already answered the question when you said that the performance was improved with the 7.5 operating. If it wasn't working (providing push to load the engine) the boat wouldn't be responding favorably when you ran it. You ARE getting work out of it. Enjoy!

My 2c.

Mark

 

[Willyclay]

Re: Two motors running at same time cause trouble?

Seed, look at the number of forum posts from TD, Mark, F R and Iwombat. Compare them to mine and you'll know that they are the "experts" on the forum, not me! However, my contribution to your question is based on my previous ownership of a 1965 Lonestar Medallion II boat that was 16.5 ft long. Here's what I know from owning that boat. It was rated at 110hp max by the BIA/OBC for whatever that's worth. It had a Mercury Mark75A when I got it and would really fly. I replaced the Merc with a 1968 Johnson 100 and the performance was about the same. The bare aluminum hull weighed only 240 pounds according to truck scales. Bottom-line is that your 33hp motor is probably marginal on your boat even if it is performing well. Good luck.

 

[iwombat]

Re: Two motors running at same time cause trouble?

I'll back myself off of that "expert" title - heh. I know just enough to be extremely dangerous in mixed company.

Really, I'd be more concerned with the safety aspects of it. If you're always running two motors in tandem that's one thing. You'll have them hooked up to the same steering and throttle controls.

Without that, if you have to make a quick stop, or shut off the motor, or any other emergency manuver, you've now got two motors to wrestle with that aren't under one set of common controls.

 

[seedpws]

Re: Two motors running at same time cause trouble?

Well the 33hp drives the boat. The 7.5 is a tiller that I control. With just the 7.5 it only goes about 5-9 MPH tops. So my wife drives the boat while I stay with the 7.5 incase I need to turn it off. I only use the 33hp when on open water as it eats so much gas and my 7.5 I can go like 20 miles on a 3 gallon tank.

So to get out of the river and to go back into the river I use the 7.5. I don't make a wake with it about half way. The 7.5 is alot shorter than the 33hp too but still pumps water ect. Its also on the side where the v isn't so deep.

So my only true question is will this cause damage to my 7.5. It is a newer motor and really good on gas and saved me many time when my 1964 75hp would break down and I was 7 miles offshore. Takes forever to get in but at least it gets me home . Also fishing Lake Erie no ocean ect.

 

[Texasmark]

Re: Two motors running at same time cause trouble?

Thanks for the attaboy but it is not applicable to me; not an xpert.....just sharing earned by doing knowledge and experiences.

You mentioned one thing that may impact the 7.5's reliability and that is you said that one is longer than the other. If you are talking about transom heights I'm assuming you are running a 20" skeg on the 33 and a 15 on the other.

You said the 7.5 is on a shallower part of the boat's transom so it may be ok.

The 33 comes from old technology (see www.old-omc.de) and has a longer shank than most engines.....like their engine for a 15" transom is around 17" from the top of the transom to the anti vent plate. This was done (my guess) by OMC because one of the water pickups is on top of the antivent plate under a cover with 2 screws holding it on (for reverse operation where the normal pickup is blocked by the exhaust housing).

The other is newer with a different water intake scheme and the antivent plate should be about level with the bottom of the boat to insure unobstructed water flow for cooling (and propulsion).......where it is mounted considering what you said about it being off to the side.
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Course at 5-9 mph tops, you aren't planing, so the location of the plate isn't all that important since the engine is running in turbulent water which touches the transom; if the plate is significantly above the bottom of the boat (where the engine is mounted) it would affect propulsion much more than cooling at those speeds.

 

[seedpws]

Re: Two motors running at same time cause trouble?

Ok with two people in the boat and a mostly flat lake the boat goes 8.7MPH (gps) with the 33hp. It goes 16.9MPH (gps) with both of them. So the 7.5 adds alot.

With 3 or more poeple the boat only does 9.9 with even both motors going. The 1973 7.5HP shouldn't add that much unless its cause it gets it up on a plain.

 

[F_R]

Re: Two motors running at same time cause trouble?

OK, the 7.5 is working as hard as it can and moving through the water at 17mph. As far as the motor is concerned, it sees itself as the same as if it were on a smaller boat that it can plane at 17mph. I can't see that as being a problem.

As for the 33, that poor thing is so underpowered that it is working itself to death at less than 9mph at full throttle. So the added oomph from the 7.5 is actually helping the 33. Even then, the 33 is working awfully hard at 17mph unless you have a pretty flat pitched prop. It probably isn't even producing 33hp because that is produced at 4500rpm. It's a good thing they are tough motors.

 

[Hoss the Hermit]

Re: Two motors running at same time cause trouble?

Just a thought, when you move back to run the 7.5 with the tiller, seems like you're gonna change the trim on the boat a lot, maybe why you get so much help from the 7.5?

 

[F_R]

Re: Two motors running at same time cause trouble?

Valid thought, however it seems he is barely able to get the boat on plane. Moving weight to the rear would only make that worse.