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Spectra 171 build


New member
Got the Spectra 171 free about 10 years ago. 17'4" long, 7' wide, 1100 lbs.
It has a pad at the bottom, so speed should be good.
Now I have enough time and money to do something with it.
It was outboard power with 150 HP. I never ran it with that.
Didn't like the concept of 150 Hp hanging on the back.
So to keep it light I will install a new Infiniti all aluminum V8 4.5 L with a twin turbo and intercooler. Weighs 550LBS only
The motor is capable of turning 7-8 grand with boost of 10-15 psi. making a WHOPPPING 600-900 PONIES.
Also I finally found an ASD6 drive with drop box 1.56 ratio. The prop shaft is 1.25 with 19 splines.
Looking around a little I ordered a 72L Velvet drive transmission good for 625 lbs ft.
I can marry the 3 together easily. Will need a driveshaft between trans and drive.
Ordered a Hill prop 15x34 5 blade for the 450R rebored the end for the threads.

A few questions have arisen.
1. Where do I place the engine for a good center of gravity.
2. How much is too much power for a small boat.
3. How fast would this machine go without blowin over.
4. Should I install trim tabs on the back.
5. Should I set the drive further back with a 6" box. The drive is already 36" long anyways.


Well-known member
You are unlikely to find anyone with knowledge of a conversion like this, most of the time when converting a boats propulsion, it happens the other direction to Outboard power instead of what you are doing. So you are likely on your own. I wish you success and would love to see pictures of your progress.

What I can say is that will be a rather long assembly in a small boat and the installation is likely to push the CG of the boat quite far forward compared to OB power, so you probably need to have the motor as close to the transom as reasonable.

But probably the correct way to do it is to bolt the power train together weigh it as a complete assembly and find it’s CG. Then you can likely find a some boat construction / engineering books for guidance on the topic and put the Spectra’s details into the formulas and see if you are close to something that works.

Best of luck


Well-known member
The other questions able how much power, what speed will it go, and maybe what is too fast.

Have you been above 75mph in a boat? Have you done it in a small boat? I’m pretty certain that if you put 900hp in a boat that size, you will never use all the power. If you start with 4-500hp and drive the boat like that for a while, you may find you do not need or want to add any more.

I would think big trim tabs are required and the boat is going to be a handful to drive.


New member
Hi Karzruz, I have Kawa 310 Jetski that does 75 mph any day and closer to 78 GPS on a good day. I suspect that the Spectra will take a whole season to get used to with 600 hp. I have done some prelim calc for center of gravity and concluded that the CG is about 5 ft forward of the transom. I have seen on some boats that run about 100 mph that most of the boat is out of the water and some guys want to trim it out as far as possible. My thought was that over 100 mph it starts to fly. I have done 120 mph in a pickup and the front wheels were NOT on the ground. The Spectra I suspect might do the same. So I want the CG as far forward as possible. Worst case, design the stringers so the engine can be moved if necessary.


New member
I've seen Carson stick 450Hp outboards on 19 footers and doing some insane speeds, with the motor moved as much as 18" back. I think it might be a bit of trail and error. In my younger years I was always looking for more power. Now I start with Big power and turn it down if need be.


Well-known member
Check the angle of the transom on the Spectra and the angle of what the new drive might have. You may have to add some wedges or something to correct it if it is different.