CONTINUOUSWAVE

I recently purchased [a 1986 Montauk 17] that had an older Evinrude V4 90-HP engine. I plan to re-power with a Honda BF90a engine which weights around 360-lbs. I'm concerned the weight of the Honda BF90a engine will cause the stern to sit too low.

I also concerned with how the BF90a will mount on the transom.

The engine splash well seems shallow and won't accommodate the [standard BIA outboard engine] mounting hole placement and spacing.

The [lower] engine mounting bolts would be going through the transom [and exiting below] the splash well—which is how the existing engine is installed. The Honda dealer thought that might be a problem.

Any insight to this situation would be greatly appreciated.

Thank you

Let me offer you some insight into your concern about engine mounting.

You topic is a frequently asked question. For a well-prepared and comprehensive answer to this frequently-asked question see

Frequently Asked Questions
Q8: How Does the Engine Mount to the Transom?
http://continuouswave.com/whaler/reference/FAQ/#Q8

For additional advice on mounting engines on older Boston Whaler boats with shallow splash wells, see the article on that topic that is pinned at the top of this forum:

Transom Mounting Hole Layout for Shallow Splash Well
http://continuouswave.com/forum/viewtopic.php?f=7&t=5808

Also read the Boston Whaler service bulletin on this topic:

Boston Whaler Dealer Bulletin 10-84
Advice on Mounting Engines on Shallow Splashwells
http://continuouswave.com/whaler/reference/bulletin84-10.html

As you can infer from the date of 1984 on the Boston Whaler bulletin, this problem has existed for about 27-years with older Boston Whaler boats with shallow engine splash wells. The onset of the problem in 1984 matches the date of introduction of a BIA standard outboard engine mounting bracket design that occurred in c.1984. I am surprised the Honda dealer selling the engine was not familiar with this problem. Perhaps he has never sold a Honda engine to re-power an older 16-footer before.

After you read (for comprehension) the several articles above on this topic, you should deduce that the solution involves mounting the new Honda BF90a engine (or any new engine) with the location of the lower pair of engine mounting holes drilled in the transom raised by either 0.75-inches or 0.15-inches from the standard BIA location. You can choose either option. The ramifications of which location chosen are explained clearly in the articles, viz., the lowest possible mounting height for the engine will then be one-hole-up or two-holes-up, corresponding to where the lower pair of mounting holes has been drilled.

Also the "older" OMC engine you have now was probably mounted using the OMC mounting hole known as the "blind hole." There are no modern engines that have a threaded mounting boss like that. If the engine you have now was mounted with the blind hole method, then the transom will need to be repaired. Fill the old hole with wood plugs cut from marine plywood. Adhere the plugs into the wooden reinforcement portion of the transom, leaving the top of the plugs lower than the hull laminate. Fill that depression with epoxy resin mixed with a low density filler. You can also tint the epoxy to match the hull color. The old holes will have very low visibility so whether or not you need to apply a finish top coat of precision color-matched gel coat resin will be up to you.

The most important aspect of repair to the old engine mounting holes is that they be made completely watertight. This concern is more important than any notion of color match to the hull.

Alternatively, the existing engine may have been mounting using large diameter wood screws or lag bolts that just tapped into the embedded wood in the transom. Using this method is no longer recommended and through-bolting is preferred. However, the holes from the old lag bolts will also need to be filled and made water tight.

Let me offer you some insight about your concern for increased engine weight.

Re the weight of the Honda BF90a engine: all Honda engines have traditionally been quite heavy for their horsepower output. The Honda BF90a engine is specified by Honda to weigh 359-lbs. When discussing engine weights, rather than offer approximations of the weight, use the actual specific weight.

The older OMC V4 90-HP engine weight can be found in a listing

Weight of Older Outboard Engines
http://continuouswave.com/ubb/Forum3/HTML/011042.html

If you use the term "weight of older outboard engines" in a search, you will be directed to that article.

An OMC V4 1632 cc platform, 85/88/90/100/115 HP in 20 inch shaft, electric start and trim-tilt weighed 301-lbs.

Now we can compare engine weights. With a Honda BF90a the engine weight will increase by 58-lbs. In order to predict where the boat's static trim will be with the new engine, you can just bring aboard some weights that can fit atop the current engine cowling and will add 58-lbs. Then you can observe the location of the engine splash well drains relative to the water line at the transom.

If the engine splash well drains at the transom are submerged with the added weight equivalent to the new engine, then you can infer the new engine weight is greater than the intended for that hull. Here I assume they were not already submerged with the old engine.

For classic era 16 and 17-foot Boston Whaler boats there was no specification for maximum engine weight. The best inference that can be made is to use the weight of an outboard engine in production at the time the boat was made that was at the maximum rated horsepower for the boat. For a c.1986 MONTAUK the maximum horsepower would have been 100-HP. On this basis, the OMC V4 90 engine at 301-lbs would be considered the maximum engine weight intended.

For many years the Evinrude E-TEC three-cylinder 90-HP engine was a very popular choice for re-powering a classic 16 or 17-footer, in part because its weight was 320-lbs, just 19-lbs more than the older OMC 90. Of course, that engine is no longer available.

For other comparisons a Yamaha F90a weighs 353-lbs, and a Mercury 90 FOURSTROKE weighs 359-lbs. The HONDA BF90a is not much heavier than competitors' engines.

If you want to avoid using an engine that weighs 359-lbs, consider moving down to a 70-HP engine. A Yamaha F70a weighs 253-lbs.

A separate discussion reviews the topic of maximum engine weight. See

Maximum Engine Weight
http://continuouswave.com/ubb/Forum4/HTML/002449.html

Let me offer you some insight into how to compensate for increased engine weight.

A method to compensate for increased engine weight is to remove other items from the transom, The best item to remove is a large lead-acid engine cranking battery. Such a battery can weigh as much as 70-lbs, so moving it out of the transom can easily offset an engine weight increase.

For advice on moving the engine cranking battery to the center console and the extremely important concern regarding the increased size of the electrical conductors, see

Location of Batteries in Small Boats
http://continuouswave.com/forum/viewtopic.php?f=9&t=5969

and be sure to read the related article

Selecting Proper Wire Size: Engine Starting
http://continuouswave.com/forum/viewtopic.php?f=9&t=4992&p=28758