CONTINUOUSWAVE

I have repowered my 1974 Boston Whaler Revenge 21 with a 2020 Mercury ProXS 200 V8 outboard. The engine is mounted two holes-up.

The initial test propellor was a Mirage 15.5 x 17p: Top speed was 50-MPH SOG by GNSS at 6100-RPM.

With an Enertia 14.2 x 18p: Top speed 52-MPH SOG by GNSS at 5800-RPM.

During both tests aboard was just 200-lbs adult. The fuel tank level was FULL.

I feel like the top end is a little light on speed. The acceleration is neck-snapping.

Give me some comments on the performance obtained.

https://hosting.photobucket.com/images/v472/sraab928/IMG_6912(1).jpg

https://hosting.photobucket.com/images/v472/sraab928/IMG_6909(1).jpg

https://hosting.photobucket.com/images/v472/sraab928/IMG_6911(1).jpg

https://hosting.photobucket.com/images/v472/sraab928/IMG_5890(1).jpg

https://hosting.photobucket.com/images/v472/sraab928/IMG_5889(1).jpg

Scott--with the Enertia, the engine is 400-RPM under WOT 6200-RPM. I would say drop pitch by 2-inches. You may gain you another 2 to 3-MPH. Then again...

Can the engine mounting go up a hole?

Typically Mercury engine can go up one-hole higher than typical due to their midsection (per LHG), and hhence the 5-hole layout rather than the typical 4-hole. That would get you pretty close all by itself.

Do you know where the torque curve is on this engine?

What make and model prop is the 13x24 stamp referencing?

Phil T wrote:Scott -

With the Enertia, you are 400 rpms under WOT (6200). I would say drop 2" of pitch which may gain you another 2-3 mph. Then again....

Can you go up a hole? Typically Mercs can go up one more than typical due to a midsection (per LHG), hence the 5 hole layout rather than the typical 4. That would get you pretty close all by itself.

Do you know where the torque curve is on this engine?

What make/model prop is the 13x24 stamp referencing?

I guess I could go up a hole - the dealer fought me on the current setup as it was "not conventional" but after I agreed to pay to fix it if I was wrong about the height they just did what I asked.

The 13 x 24 is on the lower unit - I really don't know.

I can tell you with current prop 3700 rpm is 30 mph - 0-20mph is 4 seconds - you really gotta hold on when you hit the throttle.

Phil T wrote:What make/model prop is the 13x24 stamp referencing?

The 13 and 24 are the number of teeth on the [pinion gear] and [propeller shaft] gear, and when divided gives you the ratio.

The gear driving the propeller shaft has 24 teeth and the pinion gear on the engine shaft has 13 teeth. The pinion gear is always the smaller gear. The ratio comes from 24/13 = 1.846:1

The engine has to make 1.846 revolutions to produce one revolution of the propeller shaft.

Please check the location of the upper engine mounting holes on the transom. In one of the linked images, it looks to me like the upper pair of holes is drilled more than 2-inches below the top of the transom. I am judging that based on the hole spacing of 0.75-inch. There are two empty holes above the mounting bolt. That means the center of the top hole is 1.5-inches above the mounting bolt. If the transom had the upper holes drilled at the usual position, then the top of the transom would be only 0.375-inch above the center of the top hole. In the image, the top of the transom looks noticeably higher above the empty top engine mount hole.

Please check the location of the upper engine mounting holes on the transom. Refer to this image hosted elsewhere:

https://hosting.photobucket.com/images/v472/sraab928/IMG_5889(1).jpg
As seen in the image linked above, the upper pair of holes apper to be drilled more than 2-inches below the top of the transom. I am judging that based on the hole spacing of 0.75-inch. There are two empty holes above the mounting bolt. That means the center of the top hole is 1.5-inches above the mounting bolt. If the transom had the upper holes drilled at the usual position, then the top of the transom would be only 0.375-inch above the center of the top hole. In the image, the top of the transom looks noticeably higher above the empty top engine mount hole.

Perhaps you can measure the distance to the center of the upper mounting hole in the transom from the top of the transom and report that dimension.

jimh wrote:Please check the location of the upper engine mounting holes on the transom.

The center of the upper mounting hole in the transom to the top of the transom is 1-3/4" - (measure from the inside if that matters)

sraab928 wrote:The center of the upper mounting hole in the transom to the top of the transom is 1-3/4-inch

Thanks for checking. That hole location is, if anything, a bit high, and not too low as I thought it was from the picture. The usual dimension is 1-7/8-inches.

jimh wrote:

sraab928 wrote:The center of the upper mounting hole in the transom to the top of the transom is 1-3/4-inch

Thanks for checking. That hole location is, if anything, a bit high, and not too low as I thought it was from the picture. The usual dimension is 1-7/8-inches.

I used a ruler but also had to eyeball the center of the bolt - I could be off an 1/8"

Also, and not to be overlooked: that Mercury ProXS V8 looks marvelous on your extremely well-preserved (and I image extremely well-restored) REVENGE 21.

As for top speed, heck, 52-MPH is like a rocket ship for me. The boat is 47-years-old. If you want to get another 47-years of use from it, you probably ought to keep those 52-MPH sprints limited to really calm water.

Re the ENERTIA and what might occur with a pitch down to 17 from 18:

The initial test propellor was a Mirage 15.5 x 17p: Top speed was 50-MPH SOG by GNSS at 6100-RPM.

That works out to a SLIP of 5.8. That is about what I would expect with a MIRAGEplus and that is a good value.

With an Enertia 14.2 x 18p: Top speed 52-MPH SOG by GNSS at 5800-RPM.

That works out to a SLIP of only 2.7, which is very low, almost too low based on most propellers--maybe that is the secret to the ENERTIA.

Now for a hypothetical test with lower pitch ENERTIA:
ENERTIA 17-pitch and 6200-RPM and SLIP =2.7 SLIP; calculated speed is 52.5-MPH.

The decrease in pitch to 17 from 18 for the ENERTIA is calculated to only result in an increase in boat speed of about 0.5-MPH. I don't know if an change of so little boat speed would justify buying another $600 propeller to test.

In regard to the best choice for propeller, I think either the MIRAGEplus or the ENERTIA give plenty of speed. I would be more interested in the fuel economy at cruising speed than the ultimate top boat speed. You are going to burn much more fuel at cruising speed than you will ever burn at full-throttle. Typically the engine time spent at full throttle is probably only one or two-percent of total engine time--unless you are a bass boater competing for $1-million in prize money.

When I ran the numbers I got 18" of pitch. This is based on WOT of 6200, Target speed of 56, slip of 5% and gear ratio of 1.83

I presume Scott is trimming the engine out.

For this data set:
RPM = 6200
RATIO = 1.846
PITCH = 17
SLIP = 2.7

The calculated MPH is 52.6-MPH.

There is no certainty that the absolute most horsepower from the engine is occuring at the absolute maximum engine speed of 6,200-RPM. The horsepower peak might be a bit lower in terms of RPM.

Scott, what are your thoughts on repowering your 21 Ribside?

I agree chasing a couple extra mph may be futile - the lower top speed just surprised me I guess. She is very comfortable at that speed (on calm water of course).

The boat is preserved/refreshed - not restored. She has a new fuel tank, recored cover (both done by me), all new wiring and at some point in her life the transom was redone by the original owner (boat originally came with twin motors and when it was changed to one they redid the transom).

With regards to the Ribside repower - this motor may ultimately end up on there. I haven't fully decided which boat I love more.

jimh wrote:
In regard to the best choice for propeller, I think either the MIRAGEplus or the ENERTIA give plenty of speed. I would be more interested in the fuel economy at cruising speed than the ultimate top boat speed. You are going to burn much more fuel at cruising speed than you will ever burn at full-throttle. Typically the engine time spent at full throttle is probably only one or two-percent of total engine time--unless you are a bass boater competing for $1-million in prize money.

Next time I am out I will run through the rpm ranges and report back on fuel economy and speeds.