Effect of Weight

Chip S

Chip S

Active Member
Jun 17, 2019
432
Bordentown, NJ
Boat Info
1993 Sea Ray 200 Overnighter
Engines
2023 Mercury 150 hp Four Stroke Outboard
Last year because of COVID we didn't take anyone out boating with us. It was our first year with our Sea Ray 200 OV and it would cruise at 32 mph at 4000 rpm with the two of us onboard. Two weeks ago the two of us took it out for the first run of the season and it ran at 32 mph at 4000 rpm. Yesterday we went out with another couple (4 people total) and the boat ran at 27 mph at 4000 rpm. Everything else (temperature, trim, etc.) was exactly the same. My only conclusion was that the extra weight affected the speed of the boat. I'm a little surprised it would have that much of an effect, but I haven't run a "small" boat in 20 years. My last "small" boat was a 1995 bowrider that also had weight affect its performance, so I'm not that surprised. Anyone have any experience with this?
 
Way underpropped, the rpm is just entering the power range and boat is barely planing. Transitional variations. See a boat mechanic.
 
Is the 4,000 rpm your wide open throttle max rpm?

With 4 people on board, was everyone sitting in the back and you were “plowing” and not running on “plane”?
 
32 mph at 4000 rpm is our cruising speed. Top speed with two people on board is 44 mph at 5500 rpm.
 
There are various references that state that a 100 pound increase in weight will result in a 1 mph decrease in speed. The additional 300 pounds would therefore result in a 29 mph cruising speed (32 mph - 3 mph = 29 mph). My actual cruising speed was 27 mph.

I also read that over trimming will result in a lower cruising speed. I trimmed my outboard the same as when I was running with two people, which was probably over trimmed with the extra weight. Over trim may have resulted in the additional 2 mph decrease in cruising speed. Next time I'm out with four people I'll try adjusting the trim to maximize the cruising speed. Hopefully I can increase the cruising speed to 29 mph.
 
Chip S said:
There are various references that state that a 100 pound increase in weight will result in a 1 mph decrease in speed. The additional 300 pounds would therefore result in a 29 mph cruising speed (32 mph - 3 mph = 29 mph). My actual cruising speed was 27 mph.

I also read that over trimming will result in a lower cruising speed. I trimmed my outboard the same as when I was running with two people, which was probably over trimmed with the extra weight. Over trim may have resulted in the additional 2 mph decrease in cruising speed. Next time I'm out with four people I'll try adjusting the trim to maximize the cruising speed. Hopefully I can increase the cruising speed to 29 mph.
Click to expand...
That sounds like you are on the right path. Also, the smaller the boat the more impact each 100 lbs would have. The same holds true for engine power. Lower HP motors are affected more with each 100 lb weight gain than higher HP motors.
 
Chip, I think you're overthinking this. For one, in your first post you said you were "surprised but not surprised"... you gots too many thoughts bouncing around in yer head :) Forget about reports you see mentioning weight vs mph drop - they are WAY too general to take verbatim.

What I see from what you wrote... you're able to get the RPM into the proper MAX range with your normal load. Stop. You're good. Go boating. Don't fixate on a specific speed... especially if you're using the boat's speedo and, also if you are boating in water that has current/tidal flow.
 
Last edited:
Small boat with modest HP. Not surprised speed dropped 5mph at cruise especially if the passengers are on the large size. Trim tabs may help as well.
 
Chip, read what Lazy Daze wrote, then read it again. What he says is spot on. If you fixate on every little bit of boating you are going to lose out on some of the fun of just being on the water.

Just go boating and have fun.
 
Was the wind exactly the same? Difference between say a 7mph tail wind and a 5 mph head wind will affect even though in both cases you would say it wasn’t windy.

Too early in season around here but a couple weeks of barnacle growth can have a HUGE effect.

I agree, don’t sweat it. Too many variables from week to week to make a precise science.
 
Chip, when I have everyone on board (4 adults 1 medium child) I need to have one move from bench to windshield to get on plane with my 135. Then I need to adjust trim quite a bit when she moves back to bench. Your speed for rpm sounds about right to me.

Have fun out there. I need to descale water passages and drop in a new water pump and I’m out there as well.
 
The 32 Open cruises at 26 knots with this crew aboard. That is about the same cruise speed as with just me aboard. RPMS are the same in both conditions with full fuel, water and engines turning about 3450-3500. The most noticeable difference is it does not leap up on to plane when loaded with nine adult sized people. It is no slouch, but it takes maybe 30 or more seconds to reach 26 knots. I think boat hulls have a design speed. With a clean hull, x RPMs = a y speed. HP gets you the rpms. So if you have the HP matched to the hull design, speed does not suffer with loading. Acceleration is another story. That takes torque. Diesels have a clear advantage with lots of people on board.
5FB0F849-363A-47B2-B540-807386E3FC70.jpeg
 
CNYBoater said:
Chip, when I have everyone on board (4 adults 1 medium child) I need to have one move from bench to windshield to get on plane with my 135. Then I need to adjust trim quite a bit when she moves back to bench. Your speed for rpm sounds about right to me.

Have fun out there. I need to descale water passages and drop in a new water pump and I’m out there as well.
Click to expand...

Thanks for the reality check. When I had my big inboard the loading didn't have such a profound effect. These little boats are a different breed, but I like them! :)
 
Chip S said:
32 mph at 4000 rpm is our cruising speed. Top speed with two people on board is 44 mph at 5500 rpm.
Click to expand...
Youre tach probably isn't right i do t think unless you were way under propped youd get to 5500 rpm. Is your speed off the gps or the speedometer in the dash ? These are notoriously inaccurate.
Depending on how you use your boat you may want try some different props maybe a 4 blade for more stern lift
 
Scott06 said:
Youre tach probably isn't right i do t think unless you were way under propped youd get to 5500 rpm. Is your speed off the gps or the speedometer in the dash ? These are notoriously inaccurate.
Depending on how you use your boat you may want try some different props maybe a 4 blade for more stern lift
Click to expand...

The speedometer in the dash is matching the speed on the GPS. I have an outboard so the target WOT is 5500 rpm.
 
You must log in or register to reply here.

Forum statistics

Threads
113,178
Messages
1,428,022
Members
61,088
Latest member
SGT LAT
Back
Top