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Catalina/Capri 25/250 Sailor's Forums

Catalina 25 Specific Forum

Comparing Honda 9.9 vs Mercury 15

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JanS48
Navigator

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USA
141 Posts

Initially Posted - 08/17/2021 : 21:03:10

Greetings everyone - this post is just an FYI but I thought I'd share.

I recently replaced my aging Mercury 15 [2002] with a new Honda 9.9 both motors were/are the extra long (25") shaft models with electric start.

Background: The Mercury was showing signs of age - it was developing more crankcase breather flow than I wanted to see but worse it was stalling after a long run at 3/4 throttle when attempting to slow way down. It always re-started but the behavior was getting worse with age. [a new carb did not help] Come to find out, these motors (8 thru 15hp) have that issue if running fuel containing ethanol. They can develop a minor vapor lock - this is even documented in the owners manual (which I never read). Given the motors age I decided it was time.

Hull speed: The Mercury 15 would push the boat along nicely even in a rough chop which I see often in Narragansett bay and also if heading to Block island. In calm water the hull speed would go to 7.5 knots with a large 'bump' behind the boat. The Honda being 5 ponies less did well but not as good as the Mercury - the best hull speed I could get with the Honda 9.9 was 6.7 knots. I was not too concerned about the difference in speed as I seldom run a motor wide open. Both motors got the best fuel economy when keeping the speed to around 5.5 knots or less.

In a rough chop the Honda will noticeably change pitch when hitting big waves with a notable drop in hull speed. The Mercury faired better in this area with no drop in engine speed even if the hull speed slowed some when hitting waves.

Agility: Both motors had/have 4 blade props. For some reason I'm able to maneuver the boat much better with the new Honda. When rounding up and gunning it in reverse it seems to respond faster.
Low speed cruising around Newport harbor is much improved over the Mercury with the motor being notably quieter and very rpm stable.

Fuel economy: No surprise here the Honda uses less fuel, I'd say about a quarter to a third less.

Just thought I'd share my findings. I'm very happy with the new Honda.

Jan

82 C25 SR FK
Sailing out of Newport Harbor.

Stinkpotter
Master Marine Consultant

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Djibouti
9013 Posts

Response Posted - 08/18/2021 : 06:04:50

...and I'm happy with my new one (225 hp, but never mind...)

The improvement in reverse thrust is partly due to the Honda's routing of exhaust away from the prop in reverse, for better bite. I'm also guessing the prop pitch is a little lower on the Honda. "Hull speed" is an approximate constant based on waterline length--above hull speed a boat has to essentially get out of the trough it creates between the bow and the stern, which involves enough power to climb over its bow wave. Up to that point (6.3 kts for a C-25), speed increase is reasonably linear with the throttle, but considerably more power is needed to get above that (like the 50% you used to have). So your numbers make sense. Past 6.3 you're trying to get "on plane"! (The same goes for speed under sail--you can get a C-25 there in just the right conditions, but you'll see the bow lift and the stern dig as you do.)

Enjoy!

Dave Bristle
Association "Port Captain" for Mystic/Stonington CT
PO of 1985 C-25 SR/FK #5032 Passage, USCG "sixpack" (expired),
Now on Eastern 27 $+!nkp*+ Sarge

Edited by - Stinkpotter on 08/18/2021 08:00:09

Steve Milby
Past Commodore

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USA
5851 Posts

Response Posted - 08/18/2021 : 08:42:18

It's never too late to learn something. In the post-1988 owner's manual Catalina says "An outboard engine of 7.5 to 15 horsepower should be adequate to propel the Catalina 25 at hull speed under usual conditions." C25 Hull speed is 6.3 kts.

In the pre-1988 owner's manual, Catalina recommended against using a motor larger than 10 hp because "A larger engine will not increase hull speed and may add additional unnecessary weight in the stern."

Jan says Catalina was wrong. He says a 15 hp Mercury engine can push a C25 above hull speed, to 7.5 kts in calm water. This is anecdotal, but it's safe to assume Jan must have measured his speed with either a gps or a knotmeter. We don't know whether the boat's speed was aided by a strong current, but let's assume it wasn't.

Until now, we have generally recommended no more than 10 hp, and I think that's still valid for most of us. Personally, I think my Merc 7.5 hp was ideal for most people. But it's reasonable to believe that 15 hp might gain an additional knot in speed. If the boat is used in waters with strong tidal currents, that 1 kt might be a significant advantage to the owner.

Of course there are still disadvantages to using a bigger engine, including more weight, poorer fuel economy (especially when running above hull speed), higher initial cost and perhaps maintenance costs.

One other consideration is the strength of the motor mount. In the post-1988 owner's manual Catalina says "The motor bracket manufacturer recommends that an 18 H.P. maximum engine should not be exceeded."

Steve Milby J/24 "Captiva Wind"
previously C&C 35, Cal 25, C25 TR/FK, C22
Past Commodore

Stinkpotter
Master Marine Consultant

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Djibouti
9013 Posts

Response Posted - 08/18/2021 : 20:01:08

Catalina was right�the larger engine just made it possible to exceed hull speed, at a cost in fuel and noise. Jan gave us the example!

My Honda Power Thrust 8 (same motor as their 9.9) drove Passage to hull speed well below WOT, but might not have exceeded it by much�I didn�t try. But as I reported here, it could stop Passage as if she had run into a pillow. It helps to select a motor/prop combination for a heavy displacement hull rather than a planing dinghy.

Dave Bristle
Association "Port Captain" for Mystic/Stonington CT
PO of 1985 C-25 SR/FK #5032 Passage, USCG "sixpack" (expired),
Now on Eastern 27 $+!nkp*+ Sarge

Edited by - Stinkpotter on 08/18/2021 20:12:29

bigelowp
Master Marine Consultant

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USA
1736 Posts

Response Posted - 08/22/2021 : 10:33:39

Curious -- was the Mercury 15 a 2 or 4 stroke engine and what was the weight difference compared to the new Honda?

Peter Bigelow
C-25 TR/FK #2092 Limerick
Rowayton, Ct
Port Captain: Rowayton/Norwalk/Darien CT

JanS48
Navigator

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USA
141 Posts

Response Posted - 08/22/2021 : 21:47:18

The Mercury 15 was a 4 stroke, the weight was 120, the Honda weight is 100. Speed measurements were taken using a Raymarine Dragonfly.