CONTINUOUSWAVE

Hi. [I am] new to the forum.

[You posted this in MARKETPLACE. Do not post anything to MARKETPLACE except notices of FOR-SALE or WANTED TO BUY. I have moved this article to REPAIRS and MODIFICATION for discussion. I have sent you a private message notifying you of this move--jimh.]

Last year I bought a 1976 Montauk 17 that had a Mercury 90-HP two-stroke-cycle engine that failed. I'm re-powering to a Yamaha F70 or F90.

Unbeknownst to me, the previous owner cut the transom to fit the Mercury. The Yamaha dealer is suggesting to either put a riser plate on top of the transom to get back to original transom size (for the 90 HP) or leave [the transom] alone and go with the 70. I just want to make sure the [engine I will choose] has enough power to get [the boat] on plane with four adults aboard. Also, [the boat has] a 20 gallon [fuel] tank under the seat and two batteries under the center console.

[GIve me] any thoughts on [all of these topics, that is, the transom situation, the choice of engine, and the performance to be expected when four adults and 20-gallons fuel are aboard.]

Hi Soriley and welcome to the forum,

It would be great if you could post some pictures and also take a measurement of the transom height, I find it odd that you had a 90-HP Mercury with a lowered transom.

As far as the 70 and 90 Yamaha they both come in 20-inch and the 90 can also be 25-inch. Is the dealer trying to sell you a 25 inch motor?

Also the F90 Yamaha is on the heavy side.

The F70 could be struggling with four adults and 20 gallons of fuel. Have you considered an Evinrude E-TEC 90? I just went through the same decision making process and have signed up for an E-TEC 90, being installed in the next few week.

I am sure many members on the forum will chime in with very helpful insight.

Marc-B

The most important problem to solve is the unscrupulously modified transom of the 1976 Boston Whaler MONTAUK 17 boat. Your description of the transom modification is a bit fuzzy. The transom height of the original transom should be sized for an outboard engine with a shaft length of 20-inches. I don't quite understand what would have been done to make an outboard "fit."

Just to be clear, the normal transom configuration of a Boston Whaler 17-foot hull is for the transom to be notched down in the area where the engine will mount. Here is an image that illustrates the normal transom look on a Boston Whaler 17-foot hull:



I hope you are not confused about the normal appearance of the transom.

Was the previous engine a short-shaft or 15-inch-shaft engine? That would be quite unusual. It would be very sad to learn someone cut off 5-inches from the transom. Please explain more thoroughly what has been done to the hull.

If the transom has been hacked down for a 15-inch shaft engine, you should restore it to its original configuration for 20-inch-shaft engines. You won't find any modern 70-HP or 90-HP engines with a 15-inch-shaft. I don't think they exist.

The transom restoration should be done by an experienced and qualified shop. Don't presume that every store that sells outboard engines also has the skilled people necessary to properly restore a Boston Whaler boat transom.

Engine shaft length is not determined solely by horsepower. As mentioned, you can usually choose 20-inch or 25-inch shaft lengths in modern engines in the 70 to 90-HP range.

As for choosing between a 70-HP or 90-HP engine, that is entirely up to you and your preference for reserve power. As mentioned, I would not be too optimistic about a Yamaha F70 getting a MONTAUK to jump on plane with four adults aboard. If four adults and a 20-gallon fuel load are your normal operating conditions, more horsepower may be appropriate. But there is not much value in discussing performance until we understand the present condition of the transom and what needs to be repaired.

[This article was created as a separate topic, but it appears to go over all of the same topics as the initial article posted, so I have jointed the two threads.--jimh]

Last summer I bought a 1976 Whaler 17 Montauk with a 90 Mercury two-stroke-cycle engine that blew out in the fall. I've got the boat at the dealer now just trying to figure out whether I should be 90 or 70 4 stroke Yamaha. The reason I ask the question is the previous owner, unbeknownst to me, modified (cut) the transom to make the short shaft Merc fit. Did not know this until the dealer pointed it out. He suggested putting a riser plate on top of the transom to restore it to original height. I did not get this done and he thinks we ought to put a 70 on it. Just wondering if that's enough power to get on plane with 3 or 4 adults. I have a 20 gallon tank under center seat and two batteries under center console. Any suggestions would be well appreciated. Thanks

soriley wrote: Last summer I bought a 1976 Whaler 17 Montauk with a 90 Mercury two-stroke-cycle engine that blew out in the fall.

Thanks for the additional information on the time of the purchase and precisely when the engine failed.

soriley wrote:I've got the boat at the dealer now just trying to figure out whether I should be 90 or 70 4 stroke Yamaha. The reason I ask the question...

There is not a question there.

soriley wrote:..the previous owner, unbeknownst to me, modified (cut) the transom to make the short shaft Mercury fit.

Again, this is an awful modification and should be repaired.

soriley wrote:Did not know this until the dealer pointed it out.

That is most unfortunate because it is going to cost quite a bit to fix this hacked transom in a proper fashion. It is unfortunate that you did not notice this before you bought the boat, as it has quite an influence of the value of the boat because of the rather significant cost to repair.

soriley wrote:...He suggested putting a riser plate on top of the transom to restore it to original height.

I don't endorse that sort of repair. It would be another hack. Also, where would you get this "riser plate"? Who would make it? How much would it cost? All this must be considered before such a repair could be acceptable.

soriley wrote:I did not get this done...

That's good, because it would be a poor solution. You need to get the transom repaired by a real boat craftsman. Be prepared for about $1,000 in repair cost to remedy the problem with the cut-down transom.

soriley wrote:...and [the dealer] thinks we ought to put a 70 on it.

Ask the dealer to explain his recommendation. Why is the 70-HP going to fit properly on the hacked up transom but engines of other horsepower won't? Tell us his reasoning. It is not clear why he thinks as he does.

soriley wrote:...[j]ust wondering if that's enough power to get on plane with 3 or 4 adults. [The boat has] a 20-gallon tank under center seat and two batteries under center console.

I would have the same anxiety. As mentioned above in several replies already posted, four adults, two batteries, and 20-gallons of fuel is quite a heavy load for a 17-foot boat with only a 70-HP engine. The performance of a small boat is very greatly influenced by adding weight to the boat in the form of crew, extra batteries, and heavy fuel.

soriley wrote:Any suggestions would be well appreciated. Thanks

If I were in your situation, I would:

--post on this forum a digital image that shows the present state of the boat transom so we can see exactly what has been done; this will permit more helpful replies to be made; as it stands, we can only imagine the horror of a unscrupulously modified Boston Whaler boat transom; we need to see it to appreciate the severity of the crime;

--measure the dimension of the notch in the transom and provide them to us;

--if the transom has indeed been hacked up by the previous owner, get the transom repaired properly before installing a new engine;

--ask the dealer to explain why he thinks a 70-HP engine will fit properly on the hacked transom and a 90-HP won't, and pass on that information.

To sum up my remarks, let me repeat my new questions for you:

--where would you get this "riser plate"?

--who would make it?

--how much would it cost?

--why is the 70-HP going to fit properly on the hacked up transom but engines of other horsepower won't?

Also, let me ask again that you show us an image of the modified transom so we can better assess what ought to be done.

It sounds like you're referring to a jack plate. These will get you about a 5 inch variance in height.

http://thmarine.com/th-marine-products/ ... lates.html

I agree that repair of the transom would be the proper solution. However, if money is a problem, a jack plate may work for you.

I don't recommend adding a jack plate. The transom strength has already been compromised by have 25-percent of it sawed off. Adding a jack plate will just increase the stress of the engine mounting on the already weakened transom.