Comparing the 300 HP WOT Yamaha vs. Suzuki

[baz]

Subject: Comparing the 300 HP WOT Yamaha vs. Suzuki

Over past few days I had the opportunity to cruise alongside another R-27/OB which had the Suzuki 300 HP outboard. My R-27/OB has the Yamaha F300 300 HP outboard.

My cruise partner's R-27/OB did not have bottom paint and is trailered. My R-27/OB does haver bottom paint and has been in the water for close to a year. My last diver service inspected my hull a week ago and said the hull was 'clean'.

At one point in our cruise together we decided to run the two boats at WOT, as advise by the engine manufacturers.

My cruise partner started the WOT first and I allowed it to be a good 1 mile ahead of me before I cranked up my Yamaha to WOT to follow.

This comparison is thus quite revealing as both boats were being used in WOT mode in the exact same water conditions; tide, current, wind and so on.

I believe my cruise partner's fuel tank was around 60 to 70% full. My fuel tank was 100% full, fresh water at 35 gallons and holding tank was empty.

My cruise partner had two adults onboard and I was solo with a small dog and stored gear collected up over past year.

My R-27/OB has a full cockpit enclosure.

My R-27/OB had a 15-foot SeaEagle inflatable kayak installed on cabin roof.

Both boats had a kicker motor.

Cruise partner's hull color was green and mine blue.

My cruise partner's R-27/OB was but a month old whereas mine was a full year old.

The comparison

Suzuki WOT
Top speed 41 to 42 mph
Max RPM at or close to 6000

Yamaha WOT
Top speed was 38 mph
Max RPM was 5800
Engine trim level was kept level at 3 bars on the Yamaha display.

N.B. I have separately run my R-27/OB at WOT since and the numbers I posted above were replicated.

From these results the Suzuki was the apparent winner. 😱 :?

Why do you think there is this WOT difference between the two R-27/OB models that presumably were run under almost exact same water conditions and boat weights ?

1) Could wind resistance and boat's aerodynamics explain this ?
2) Aerodynamic force is influenced by speed squared, so could be significant at 40 mph.
3) Could my cockpit's full enclosure and roof top kayak be responsible for my WOT results.
4) My 5800 RPM seems low as the published WOT number is 6050 (I believe).

As an aside, I noticed that at all times my Garmin chart plotter was displaying a MPG value significantly larger than what the Yamaha display was showing. For example Garmin would show 2.01 MPG whereas the Yamaha would be showing 1.6 MPG. I will also mention that the Yamaha's MPG would fluctuate rapidly up and down such as 1.4, 1.6 1.9, 1.6,1,2, 1.6, etc whereas the Gamin's MPG would fluctuate by much smaller amounts. This IMO is a significant difference and wonder which one is the more accurate ? Does something need to be calibrated to resolve this difference.

 

[Red Raven]

Hi Baz,

Interesting! Good test!

Our R27 Classic achieved the rated 4000rpm when it was new. It now only gets to about 3850 to 3900(at best) under the same conditions and same aerodynamics and with brand new bottom paint. I attribute the difference to weight of gear that gets added to the boat over time. Boats are like people, they both get fat!

In your case, here are my thoughts.
- You don’t really know the weights are the same without weighing each boat. Over the year you have had the boat you may have added gear and thus weight.
- Bottom paint vs. no bottom paint. I suspect bottom paint may slow the boat a bit. I don’t know if it could be that much or not.
- Clean bottom vs. really clean bottom. A visual inspection of “clean” is not the same clean as a trailered boat that has never had any buildup.
- Did your boat use to get 6000rpm WOT when it was brand new? If it did, then that should help explain.
- I would guess that the difference is a combination of the above items plus potentially the aerodynamics as well. You could test the aerodynamic effects by doing a test with and without the Sea Eagle on top and with and without the side panels zipped on.
- It is possible that the engine performance changes a bit over time as well.
- Also I am assuming that the props are the same?

Curt

 

[Chimo]

Hi Baz

I think you have detailed the possible reasons perfectly. Aerodynamics are going to play a very real role and the Bimini as well as anything on the roof are going to be significant. Weight differences are hard to assess without being on the scales but I suspect some difference will exist.

I'm interested as to how you trim the boat. I'm very familiar with deep V hulls as well as the semi-displacement 23 C-Dory. The latter boat may well be the better comparison as I don't know what the dead rise angle is on the 27 with outboards but regardless the two hull types responded pretty much the same. My procedure is to trim motors down to get over the hump and well on to plane and then trim out as I increase throttle. As I achieve WOT I trim out further until the boat just begins to porpoise in smooth water. I then pull back a little until the porpoise motion stops. That should be the optimum attitude for the hull. On the C-Dory the speed difference gained by the above was about 2 mph.

Though your diver reports your hull as "clean", all things are relative and I suspect this may well be the real culprit. I am fortunate that I keep our boat in the boathouse which means I collect no growth on the bottom and only minor grunge at the stern. Nevertheless, after 6 months in the water the bottom has still picked up a coating of slime. In the summer time I can even tell the difference when the boat has not been run for about 2 weeks. After cruising for about 30 minutes some of the slime has worn off and my speed will of picked up a little for a given throttle setting. This showed up really clearly back in July when I hauled the boat out, wet sanded lightly what looked like a clean bottom and applied a couple of coats of new bottom paint. For me, WOT is given as 3000 RPM. When the boat was new I could achieve 3020 RPM but this had dropped off to 2960 prior to the haul out. With the new bottom paint and with no change in weight I was back up to 3020 RPM and a 5% increase in speed.

Hope this is interesting.

 

[Pilotnavigator]

There was no mention of prop size and pitch and design configuration. This can make a difference.

 

[ixlr8]

My 27OB w/Yamaha with 2 people, full fuel, full water, empty black water tank, no bottom paint, trailer-ed w/clean hull would do 41-42 mph WOT. The boat has been in the water at my local marina for 6 weeks, with no bottom paint and there is a bit of growth on the bottom. Now WOT only gets me to 37-38mph with 2 people, full water, empty black water tank, fuel tank was less than 1/4 full.

 

[baz]

My 27OB w/Yamaha with 2 people, full fuel, full water, empty black water tank, no bottom paint, trailer-ed w/clean hull would do 41-42 mph WOT. The boat has been in the water at my local marina for 6 weeks, with no bottom paint and there is a bit of growth on the bottom. Now WOT only gets me to 37-38mph with 2 people, full water, empty black water tank, fuel tank was less than 1/4 full.

Jim: It seems we are in the 'same boat'. 🙂

We both have lost some WOT performance with similar hull condition and full cockpit enclosure. 😱

The Yamaha performance Bulletin shows WOT giving 41.4 mph engine running at 5900 RPM. Hmmmm, so does the 100 RPM equate to some 3 to 4 mph ?

Shouldn't the engine run at 5900 at WOT regardless of hull condition and the aerodynamic resistance of a full enclosure ?

No matter, I'm not unhappy with 38 mph. I cannot keep the hull clean at all times as some can having boat out of the water and no bottom paint, plus I much prefer the full enclosure vs. the extra 3 to 4 mph. 38 MPH is fast enough for me and can really only be attained in decent surface water conditions.

My last run of about 5 miles last Sunday was with a head wind and maybe a 1 foot chop with close to WOT and it gave me a decent 35 mph. The speed is nice to have at times for sure in this R-27/OB. 😀

 

[knotflying]

Shouldn't the engine run at 5900 at WOT regardless of hull condition and the aerodynamic resistance of a full enclosure ?
😀

The answer is no, Once you exceed the perfect conditions that resulted in the published numbers the WOT will decrease somewhat. Growth I weight are some of the factors.

 

[baz]

Mike: I agree. 🙂

One of the aspects of having the outboard motor is that coming from an inboard Diesel engine where the throttle setting controls the engine's RPM primarily the outboard's throttle position acts differently.

I can set the throttle at some position and the RPM will start off at some value and slowly increase as the boat hunts for its planing attitude. As the boat gets up onto plane the RPM will increase quite rapidly and eventually settle down to a constant RPM.