CONTINUOUSWAVE

[Moderator's note: this thread has been moved to the PERFORMANCE from a forum whose topic (ironically) is about how to use the website. The model name of the boat has been corrected.]

I need advice that verifies my opinion that the Mercury 90-HP FourStroke engine [I bought new from a dealer] is not the correct engine for an old 1969 KATAMA 16 boat. I need some sort of proof to share with the dealer to encourage a conversation about a refund of the purchase of the Mercury 90-HP Fourstroke engine which I would apply toward buying a different new engine.

BACKSTORY and NARRATIVE of a RE-POWER

In 2026 [my 1969 KATAMA 16 which I have owned since 1970] needed a new engine. My dealer chose a Mercury 90-HP FourStoke. With the much heavier Mercury 90-HP FourStroke engine the 1969 KATAMA 16 is not handling well. The stern sits low in the water, the acceleration is not good, the boat does not get onto plane, and it does not perform well.

My concern is the Mercury 90-HP FourStroke engine is too big and too heavy for the hull design and transom of the 1969 KATAMA 16 boat. I also worry that the Mercury 90-HP FourStroke engine could damage the transom on the KATAMA. I placed telephone calls to the outboard engine dealer that sold me the engine to discuss my opinion that the Mercury 90-HP FourStroke is not the right engine for this boat; the dealer does not return my telephone calls.

Q1: is the Mercury 90-HP FourStroke the dealer sold to you a current model in production?

Q2: what is the shaft length of the Mercury 90-HP Fourstroke engine?

Q3: what horsepower rating was the prior two-stroke-power-cycle engine in use when you were happy with performance?

Q4: what propeller make, model, material, number of blades, diameter, pitch, is on the engine?

Q5: what is the gear ratio of the Mercury 90 FourStroke

Q6: do you have any actual on-water performance data to present to readers and to the dealer that document the boat speed, engine RPM, and fuel economy at various engine speed settings from 2,000 to 6,000-RPM, and particularly the engine speed and boat speed obtained at wide-open-throttle with the new engine?

Absent the information requested above, assessing your request for a refund is going to be very difficult for the dealer and for readers of this thread.

Much of the "poor" performance you experience would be due to a very poor choice of propeller.

If you supply all the missing information, readers can better assess your situation and offer advice. Without all the missing information, readers can only make blind stabs at advice.

All modern four-stroke-power-cycle outboard engines tend to be physically larger and heavier than legacy two-stroke 90-HP engines. Even the modern two-stroke-power-cycle direct-injection E-TEC engine at 90-HP rating was 320-lbs.

The lighter 90-HP modern engines now range from 344-lbs (Suzuki DF90A) to the heaviest one, a 392-lbs (Tohatsu MFS90AETL).

The only approach to get a significantly lighter engine that you have now (which I suspect weighs around 360 to 363-lbs) would be to move to a lower horsepower rating, like a 70-HP engine. But if you do not like the acceleration and boat speed from a 90-HP, then you probably won't like the performance from a 70-HP engine.

The Yamaha F70 (current model) weighs about 253-lbs. Switching to that engine would take 110-lbs off the transom.

Typically when these older 16/17-foot hulls are re-powered, as much weight as possible is moved out of the stern of the boat. The boat battery is typically moved to amidships and often under the helm console. Any on-deck fuel tanks are moved out of the stern and to somewhere forward, even to forward of the helm console.

All performance of a power-driven vessels is always in proportion to the ratio of power-to-weight. On a 57-year-old Unibond hull there is always a chance that the hull has increased in weight over the years due to ingress of water into the interior foam between the hull and liner.

And crew weight has a very big influence on performance on boats with relatively light hulls, such as an open 16-footer like a KATAMA. So if your use of the boat includes carrying four people that all weight over 200-lbs, they are going to have a significant effect on acceleration and top speed compared to one-occupant who weighs perhaps 170-lbs.

I will [in the near future provide answers] to questions Q1 to Q6.

With the Mercury 90 FourStroke, I worry about permanently damaging the boat's transom. One concern that I can't seem to get an answer, and I only have opinions on is:

Q7: is the Mercury 90 FourStoke engine too heavy for the transom of a 1969 KATAMA?

Q8: is there any published data on the maximum transom weight for a 1969 KATAMA?

I have read [somewhere unspecified that the maximum allowed engine] weight [for a KATAMA 16 is around] 300 to 330-lbs.

I have read there are work arounds [that permit heavier engines to be used] with transom supports.

Joey wrote:Q7: is the Mercury 90 FourStoke engine too heavy for the transom of a 1969 KATAMA?

To assess the weight we need to know how much the engine weighs. So far there is just an assumption about how much the engine weighs. You need to find the "dry" engine weight as specified by the manufacturer for the particular model of 90-HP engine and shaft length you have. It is common with Mercury that they might have two different gear case options and each has a different weight.

Also with regard to engine weight, the specified weight is without any oil in the engine, so more weight should be added for the quantity of oil in the oil sump. The engine weight does not include a propeller, propeller hub, and coupling inserts. So add more weight.

Joey wrote:Q8: is there any published data on the maximum transom weight for a 1969 KATAMA?

In the Boston Whaler literature from 1969 I do not see any specification or limit given for the transom weight. Boston Whaler only began to publish specifications for maximum transom weight about 20-years-ago. I cannot give a more precise start to that epoch because I have not really researched it. But certainly in 1969 when the boat was being sold as a new boat, there was not a factory specification provided about engine weight.

About the only inference that can be made about engine weight is to look at the horsepower rating for the boat in 1969. You can find that information in the REFERENCE section article on

DIMENSIONS and PRODUCTION INFORMATION
https://continuouswave.com/whaler/refer ... #hulls9-17

The 16-foot hull with remote steering and throttle controls was rated for a maximum of 100-HP. Using that figure you can look up the weight specifications for older outboard engines of that era. Information about the weight of older outboard engines has been collected at

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

Many Boston Whaler boats were typically rigged with OMC engines, so the weight of an OMC 90 or OMC 100 would be a reasonable value of engine weight that the boat would be rigged with back in 1969.

If you look at the collected information on OMC 90-HP or 100-HP engines, you will find that a 20-inch-shaft V4 engine would weight 301-lbs.

Now when you finally figure out how much your Mercury 90 FourStroke engine weighs, you can make a comparison.

Joey wrote:I have read [somewhere unspecified that the maximum allowed engine] weight [for a KATAMA 16 is around] 300 to 330-lbs.

Making allusions to something you recall having read but not giving a proper citation of the source is not a particularly convincing way to produce the sort of "proof" you want to present to the dealer to get him to refund your purchase price of the engine you bought.

Joey wrote:I have read [somewhere unspecified] there are work arounds [that permit heavier engines to be used] with transom supports.

Again, recalling that you read something without any citation of the source has little probative value in swaying the dealer. Also, the term "transom supports" is very nebulous. You need to explain what was meant by "transom supports" and how to install them and what effect they have on the maximum engine weight number you want to establish.

Joey wrote:Q7: is the Mercury 90 FourStoke engine too heavy for the transom of a 1969 KATAMA?

One method to assess the engine weight on any Boston Whaler boat is to observe the position of engine engine splash well drains relative to the waterline at static rest in the water. Under the assumption that a "drain" is supposed to removed water from the splash well, if the outlet of the transom splash well drains are submerged below the water, then they are no longer drains, but are allowing seawater to continually flow into the splash well.

If the transom drain outlets for the splash well are just at the water line, that is common, but for them to be entirely submerged and an inch or two or more below the waterline, that is an indication that weight should be removed from the stern of the boat.

In terms of getting the selling dealer to take back the engine you bought and refund the purchase price or apply the purchase price toward another engine, I would think the possibility of that occurring depends on many factors. For example, if you were a long-time customer of this dealer and had bought several other engines from him, so you were perhaps in his mind a really "good customer", then the dealer might be willing to work out some arrangement as you propose.

Unfortunately, once a new, unused, fresh-out-of-the-shipping-crate outboard engine is sold, then installed on a customer's boat, and the customer uses the engine, perhaps accumulates a few hours of running time with the engine, that engine is no longer "new", and the chance that someone else would buy the engine at the original sale price becomes rather slim.

About the only reasonable and provable basis that the dealer erred in putting that Mercury engine on your boat would be if the manufacturer of the boat had clearly stated there was a maximum engine weight permitted.

In 2026 your c.1969 boat is 57-years-old, and there is no reasonable basis to assume that the structural strength of the of the hull and transom have somehow INCREASED with age. The opposite is much more likely, and particularly for the transom. Because the transom strength depends significantly on the added wood reinforcement, if there is any sign of water having gotten into the wood and caused rot, then the assumption that the transom is not as strong as it was 57-years ago is more likely than it got stronger.

Q9: have you tried different engine mounting heights and or propeller selections? Handling problems can often be solved by selecting the best of these two variables.