73 Aquasport 170 motor issues

[Wallen]

73 Aquasport 180 that I am slowly rebuilding. I'm told I have bought the wrong motor for it....2002 Yamaha 115 hp 4-stroke. Is this so? Is shaft ...24" ...to long?
Is motor to heavy? Any info would be of great help
 I'm new here but hope to spend much time as I restore this boat. Thank you!

[RickK]

I think you mean 170. If so, they are very sensitive to weight and a four stroke on the aft will put the scuppers under water for sure. The transom is set up for 20" outboards.

[wingnut]

If you’re redoing the transom, XL (25” shaft) is not bad since you can build it taller. Just make sure you’re measuring it right - clamp to cavitation plate should be your transom height.

115 4 stroke will sit a little low but I would not say it would be dangerous, especially if you’re raising transom 5”. Just don’t put a big fuel tank or bait well to aft.

Ditto Rick, just don’t expect it to be self bailing. I have a 270lb Suzuki DT85 on mine and it is self bailing, but barely.

[boatnamesue]

73 Aquasport 180 that I am slowly rebuilding. I'm told I have bought the wrong motor for it....2002 Yamaha 115 hp 4-stroke. Is this so? Is shaft ...24" ...to long?  Is motor to heavy?

I have the ‘76 170 model with a Yam ‘98 115hp.  Your 4 stroke is same weight as my 2 stroke.  This motor is not too heavy and in my opinion is the perfect HP for this hull to achieve maximum hull performance.  The motor will drop the scuppers to just above water (salt), but I keep them plugged anyways. 

However....Bad news is the 25” shaft is too long.  You need the 20” or you will certainly experience significant performance issues, both motor and hull.  I don’t really see a workaround if you hung the 25”, unless you enclose the transom, hang the motor on a hydro jack plate, and mount the plate higher up towards top of transom to compensate for the +5” shaft length. 

[Wallen]

Soooooo...it seems I have a 170! My bad...
After looking all over internet I see many that had a 115hp Yamaha on them...I have a 2005 115hp Yamaha 4stroke  and am now wondering g if it will work? It's a long shaft and am looking into changing to short (my wife runs office at a local marina on Lake Winnipesaukee). My other thought is to try to sell/trade/barter it for a smaller motor...not sure. There is a transom height extender I've seen ...is this an option? Thanks for all the info and help!
I'm also going to remove onboarding gas tank to determine why they had motor hooked up to a separate tank on deck. Motor was a Johnson 90 VRO that mice destroyed.

[wingnut]

Long shaft is 20" so you should be good. That length is measured from the inside of the clamp (where it hits the top of the transom) to the cavitation plate (the big flat flange that sticks out from the lower unit). The cavitation plate should be roughly even with the bottom of the keel. Take a measurement before you switch. You will not go anywhere with a short shaft (15"). Your prop will just churn air. If you measured your 24" from clamp to propshaft, you almost certainly have a 20" shaft.

If you do indeed have a 25" (extra long) shaft, there are options but all require some work and careful planning.

What kind of rebuild do you have in the works? Typical transom/sole etc.?

[RickK]

Ask your wife to find one of these engines for you - the perfect engine for a 170. I had one and it clamped onto the transom but boy would she run. A light engine - the tower of power. Looks to be twins on an Aquasport in the ad.
https://www.ecrater.com/p/23405748/1975-mercury-marine-color-ad-150-hp

[boatnamesue]

Like wingnut said, before you go swapping engines I'd first confirm the shaft length.  This is easy to do because the model number indicates the shaft length along with other motor specifics.  The 1st letter after the hp is type of tilt/trim, 2nd is shaft length, 3rd is controls type, and 4th is year.  The model number is stamped on a plate located on the bracket port side.

Yamaha Model Reference Chart
Shaft Length
S = 15"
L = 20"
X = 25"
U = 30"
 
For example, my motor model number is S115TLRW so the "L" indicates the shaft length of 20".  Since your motor is a 2002 I'm guessing your model number is F115TLRA or F115TXRA

[Wallen]

Wellllll....it seems the 2005 Yamaha 115 is a 25" shaft length and the 90hp Johnson VRO that came with it is 20".
Wondering whether a jack plate to get it to 20" issomething I should do? There seems to be many who have a
115hp on their 170s put thinking my 4stroke might be to heavy?
I'm about to remove onboard gas tank to determine why they had on deck tank and will check interior frame, foam, etc. I hope there were only hose problems but not sure what type of tank I'll find.
For what its worth... I'm 64 yrs young, grew up on Lake Winnipesaukee in NH, will be spending majority of time on ocean, am more of fisherman than boater and will do much Striper fishing, possible boat will spend time on west coast FLA in winter and NH/ME coast in summer.

[wingnut]

Overall weight should be fine, just don't expect it to be self bailing. I can't speak to whether you can get a jackplate to work... I'd be concerned about putting more weight farther aft, but possibly it would be ok. However, if you do go that route, make sure your transom is VERY sound. Hanging a motor offset puts extra stress on the transom. If your 1973 transom is original, it could be on its last legs. Look for a telltale crack along the aft edge on the top.

If you do have to replace your transom, it's a great opportunity to raise it 5" and forget the jackplate.

[Wallen]

The jack "plate" I am looking at raises the transom vertically instead of offset like some. I'm looking into how it mounts and making sure it doesn't put all the weight/torque on the upper few inches.

[Mwar410]

You can get a Bobs manual plate that has different offsets depending on how you install it. I think the min offset is 2.5” and a max of 5” . It’s  a 2 piece plate that mounts identical as the motor.

[boatnamesue]

For your reference, here is what a Yam 115hp looks like hung on the 170.  I'm fairly certain the transom dimensions of my '76 170 is the same as your '73 170.  Despite the difference in 2 stroke/4 stroke Yam models, the overall weight (400lb) and bracket dimensions are the same.  And as you can see here, you could not safely install the bracket on the transom any higher....or at least you wouldn't want to.  If considering a jackplate, which I've brainstormed a number of times, the issue you'll run into with the 1970s 170's is the rigging trough on starboard side.  With a jackplate the motor will move vertically, which in turn will move the rigging vertically and thus unseat the rigging trough hatch.

As wingnut noted and I can confirm, there is no weight issue w/the 115hp hung on this hull and moving weight aft will ABSOLUTELY change performance.  I have 13 gallon above deck baitwell and when filled, hull performance is sluggish compared to when empty.  And this effect is dramatically enhanced when I have a full fuel tank and full baitwell.  If I were to install a jackplate I would also install trim tabs to better control the bow rise.

There would be a lot of "what if's" installing the 25" shaft....if possible, I'd sell/swap the motor you have for the 20" so these "what if's" go away.  Good luck!

[RickK]

Another thing to consider when using a jackplate is the steering cable. In Jason's pic above of his Yammy you can see the steering cable just inside the transom. Using a jackplate will entail getting a longer steering cable and somehow getting the stiff cable up and over the transom and have it still work.
I needed to install a jackplate on my 170 due to miscalculations of where the engine mounting bolts would come through the transom. The bottom bolts ended up coming through the transom where the floor met the transom - DOH!!  So I found an used electric 6" offset jackplate. This caused another set of woe's as I also had hydraulic steering on the engine. The jackplate was mounted lower than normal so I missed the floor with the bottom bolts and I was able to mount the engine high enough so that at the lowest point of travel, the engine was at the perfect height with the cav plate at the bottom of the keel - YAY. Now came the unforeseen troubles - 1) the hyd steering wouldn't fit in the notch of the transom and 2) I couldn't tilt the engine forward, even raised all the way, without sheering off the couplers where the hoses came into the hyd steering. I needed more setback to accomplish this.
I bought a adapter mounting plate for the PowerPole which was 1/2" thick and still needed 2 more inches to clear the transom. I found 2" thick solid aluminum spacer blocks from a company and combined it gave me the clearance I needed to raise and tilt the motor forward, out of the water. I have about 8.5" of setback total now.




You can see the black Powerpole adapter plate in this pic.


How does it run now? During my rebuild I planned moving 3 batteries into the console and the console was modified which added weight forward in the boat, so it sits as it should in the water. The scuppers were drilled higher in the transom during the rebuild also, so it self bails still. I do put plugs in while launching.
After I launch the boat into the water, I tilt back the engine and lower it all the way down. The boat handles a little differently now with the engine set back but not squirelly. This pic is from break-in of the engine while running a portable tank with double oil mixed into gas.

[Wallen]

All good info, thank you!
I have another possibility and looking for any advice.....
Motor has an obvious extension block showing that it was probably changed from 20" shaft to 25.
What parts would I need to change back to 20"?
I should add that I have access to a coue of marine mechanics as well as my wife being a manager at a marina on Lake Winnipesaukee.
I also have an offer to sell at everything I paid for it yet am struggling to find same motor/ size at 20" that isnt a 2stroke one older.
Thank you in advance!)