CONTINUOUSWAVE

Q1: what is the expected maximum boat speed for a 2003 130 SPORT with a 40-HP two-stroke-power-cycle engine?

BACKSTORY
In Spring 2025 I bought a 2003 130 SPORT with a Mercury 40-HP two-stroke-power-cycle engine. I installed new spark plugs and a new water pump.

With only me (220-lbs) aboard, the maximum boat speed is 27-MPH (measured by GNSS receiver) at 5,500-RPM. I am not complaining, and this is fast enough for the boat’s intended purpose, but people say these boats do 40-MPH all day long.

I am concerned that this 2003 130 SPORT is heavier than it should be. Before I begin to look for wet foam, and I want to know:

Q1 (repeats): what is the maximum boat speed a 2003 130 SPORT [with a 40-HP 2-cylinder Mercury two-stroke-power-cycle engine] has achieved?

The boat sits with the red line a little above the water line, and it is even all the way around.

The engine peak dry cylinder pressures is greater than 90-PSI on both cylinders.

The engine vibrates a little. I figure this vibration normal for all two-stroke-power-cycle engines.

Q3: could the present low boat speed and the vibration be caused by a problem with the carburetors?

Please offer advice.

The best way to compare your boat's top speed to what is expected would be from a factory performance report. However, I doubt there was such a report published by Boston Whaler.

Regarding the power output of the 40-HP engine, the engine being able to accelerate under load to 5,500-RPM is an indication that it has reached its normal engine speed at which it is producing its rated horsepower.

Regarding your worry that the boat is heavier than it should be: the first problem in that notion is to accurately know the original hull weight, the current engine's weight, and all the gear and fuel you had aboard during your performance test that produced a top speed of 27-MPH.

As for the hull weight, a short article from BOATINGWORLD notes the weight of a 2003 130 SPORT is 600-lbs NET. I presume this does not include the engine.

A Boston Whaler specification sheet for 2010 130 SUPER SPORT gives the dry hull no engine weight as 640-lbs

In 2022 Boston Whaler literature, a 130 SUPER SPORT is noted as having a dry weight (without engine) of 695-lbs, and although that is a different model, it is indicative that these 13-foot hulls might weigh somewhere between 600 and 700-lbs.

Using that range for the bare hull weight, please give us the weight of the engine and any other gear and fuel you had aboard in the speed test.

Until we get your estimate of the total weight during the performance test, I will consider the engine weight for a TWO-CYLINDER (based on your mention of "both cylinders") Mercury 40-HP engine in 2003 was as little as 165-lbs (based on a GROK.COM AI-assisted search), and that other gear and fuel aboard totals 75-lbs. Then adding your 220-lbs, I get a total boat weight as follows:

HULL = 650
ENGINE = 165
GEAR and FUEL = 75
CREW = 220
TOTAL = 1,110-lbs.

Now the boat speed will be estimated using the Crouch Planing Hull Speed Prediction method (as I have implemented in a calculator), with the following inputs

HP = 40
LBS = 1110
HULL COEFFICIENT = 180 (common for Boston Whaler hulls)

The predicted maximum boat speed on plane is then 34-MPH.

Your observed top speed is only 27-MPH. Plugging that value MPH into the Crouch formula, we can estimate the total weight would be 1780-lbs, which is an enormous variance from the estimated weight of 1,110-lbs.

Alternatively, using the estimated weight 1,110-lbs and observed speed 27-MPH, we can predict the horsepower. That calculation gives 25-HP. If this were true, the engine would be producing significantly less power than expected.

To obtain a boat speed of 40-MPH as you have come to expect, with only 40-HP the total boat weight would need to be only 810-lbs, which is not possible for a 2003 130 SPORT with a 40-HP engine and an adult person aboard.

Note that in small boats the maximum boat speed will vary considerably depending on the total weight of the boat.

Also a two-cylinder 40-HP engine would probably tend to run with some vibration because of only having two cylinders.

Thenj0esaid wrote:The boat sits with the red line a little above the water line, and it is even all the way around.

What is "the red line" that you are using as a reference point, and how far above the water line is "a little"?

A good indication of proper hull weight and proper static trim is to use the engine splash well drains on the outboard side of the transom and their position relative to the water line. In normal weight and trim, the engine splash well drains should be above the static water line, or just slightly submerged so that only the very deepest portion of the engine splash well holds water. At no time should the engine splash well drains at static trim be submerged to a depth that causes water to overflow the engine splash well.

I have a 130 [SPORT] with a 2014 Mercury 40-HP FOURSTROKE 3-cylinder fuel-injection engine with 1:83 gear ratio.. The top boat speed is 31-MPH at 6,000-RPM, with just me aboard and 6-gallons of fuel, using a Mercury SPITFIRE four-bladed 10-3/8-inch-diameter x 13-pitch propeller. This propeller has produced faster acceleration from a standing start and higher maximum boat speed than the factory-rigged Mercury BlackMax three-blade 10-3/8-inch-diameter x 14-pitch. The BlackMax seemed sluggish getting the boat on plane and only provided 28-MPH top boat speed.

The boat performance was surprisingly propeller-sensitive. Usually you can choose a propeller for better top end boat speed or better acceleration, but that four-blade with one-inch lower pitch was actually superior in both ways to the standard propeller that Boston Whaler combines with the Mercury 40-HP engine.

ASIDE
I can't speak about the two-stroke-power-cycle engine.

Mattycat wrote:The boat performance was surprisingly propeller-sensitive.

I don’t think there is anything in particular about a 2003 130 SPORT and a 40-HP engine that makes the choice of the propeller to be especially more influential on boat performance than will occur with other hulls with other engines. Getting the best performance from a boat and engine combination generally is always very much affected by the choice of the propeller.

In many four-stroke-power-cycle engines, the maximum power output occurs very close to the maximum engine speed allowed. For those engines the propeller should often be selected so it allows the engine to accelerate to its maximum allowed engine speed, thus producing the maximum power output.

In the example above, picking a propeller that allowed the engine to accelerate to 6,000-RPM, probably the maximum engine speed recommended, let the engine produce the most power output it could. With more power from the engine, the boat speed should—and did—increase.

Mercury literature about their SPITFIRE propeller extolls these virtues:

Thanks to the most aggressive geometry of any aluminum prop in its class, the SpitFire delivers exceptional overall performance, including high top speeds, confident control in turns, and 16 to 25% faster acceleration than a comparable three-blade prop.

It seems like you got exactly what you paid for in that SPITFIRE aluminum four-bladed propeller. That it worked well for you is probably not unique to just your hull and engine combination.

As for Boston Whaler not rigging the boat with the SPITFIRE propeller, that particular family of propellers may not have been available when the boat was delivered from the factory.