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Q1: will a 1980 SPORT 15 with a c.2010 Mercury 30-HP FourStroke be under-powered [for the uses described below]?

BACKSTORY
For ages I have been searching for my first Boston Whaler boat to buy. I now have the option to buy
a 1980 SPORT 15. This would be my first boat. The boat condition is "decent." The engine is a c.2010 Mercury 30-HP FourStroke. I assume that with only 30-HP the SPORT 15 will be under-powered for my use, which, for the most part, will be ferrying me and my dog to a cottage. Occasionally I will pick up friends and family and ferry them over.

I do not care about top boat speed; I just want enough power to get the hull on plane.

ASIDE
I have been consuming every bit of written knowledge and motion picture presentations on the classic 13- and 15- footers. I have a ton to learn. I am thankful for the wealth of information on this site.

JBroat wrote:Q1: will a 1980 SPORT 15 with a c.2010 Mercury 30-HP FourStroke be under-powered [for the uses described below]?

According to Boston Whaler, a the 15-foot hull (and the SPORT 15 would be the lightest model) should be able to accelerate onto plane with an engine of 20-HP at minimal loading. This suggests that a SPORT 15 with one (180-lbs) person, a (small) dog, and a (light) 30-HP engine should be able to get on plane.

Of course, more horsepower means faster boat speed, faster time to plane, and more reserve to carry heavier loads. The notion of carrying more adult people aboard a small boat like a SPORT 15 is perhaps more influential than you might think. If just you and your dog total, say, 200-lbs, and you decide to pick-up two large people, each weighing 200-lbs, then the crew weight will be tripled to a total of 600-lbs from just 200-lbs. The extra 400-lbs will affect performance more than you might think, as the ability to reach a certain speed is always related to the ratio of power-to-weight. If the bare hull is 500-lbs, the engine is 150-lbs, and gear and crew are 250-lbs, then the total boat weight is around 900-lbs. Adding 400-lbs increases the weight to 1,300-lbs, or a weight increase of 1.44-time. To keep the performance the same as with the 30-HP and just dog and you, the horsepower must increase by the same amount, 1.44 x 30 = 44-HP. The greater the weight increase becomes in terms of how much it raises the total boat weight, the more horsepower will be needed to maintain the same boat speed. If the power is not increased by 1.44-times, the the boat speed must decrease in proportion to the square-root of the power-to-weight ratio.

You do not give any information about the distance of your ferry trips. The distance will affect how significant a change in boat speed might be. For example, if the distance to be traveled is only 0.5-miles, then the duration of the trip will not be much changed if the speed changes by a certain amount.

Let's say the boat can plane at 25-MPH, and the trip is 2-miles. The duration will be 5-minutes. If the speed increases to 35-MPH, the duration of the 5-miles trip decreases to 3.4-minutes. You saved only 1.5-minutes by going faster.

On the other hand, if the trip is 15-miles, the trip at 25-MPH will be 36-minutes. If the speed increases to 35-MPH, the duration of the 15-mile trip decreases to 26-minutes. You saved 10-minutes by going faster.

COST TO REPOWER
The best way go get a newer engine at a moderate cost is to buy the newer engine already on the transom of a boat and let the seller eat the depreciation cost of his original purchase of the new engine. For example, if you wanted to go faster and bought a 40-HP engine, you would discover the cost of new 40-HP engines these days is just crazy.

I asked GROK.COM for prices. Here is the answer:

How much is a new 40-HP 20-inch-shaft four-stroke outboard engine in the USA?

The price of a new 40-HP, 20-inch-shaft, four-stroke outboard engine in the USA varies by brand, features, and retailer, but based on available information, here’s a breakdown of estimated costs for popular brands as of my last update in July 2025:

Yamaha F40LA: Approximately $7,130–$7,500. This model is noted for its lightweight design (around 214–218 lbs), electronic fuel injection (EFI), and power trim and tilt. Prices can vary depending on included accessories like digital gauges or propellers.

Mercury 40HP FourStroke: Prices range from about $6,500–$7,500, depending on the specific model (e.g., 40ELHPT SeaPro Command Thrust). Features include EFI, a high-output alternator, and suitability for aluminum boats.

Suzuki DF40A: Around $6,000–$7,000. This model weighs approximately 229 lbs, offers EFI, and comes with a 7-year limited warranty for purchases made between May 23, 2025, and July 3, 2025.

Honda BF40: Estimated at $6,500–$7,500. It’s the lightest in its class (around 214 lbs), with features like Lean Burn Control for fuel efficiency and a 5-year warranty.

Tohatsu MFS40A: Priced around $5,800–$6,500, this is among the lightest in its class (209 lbs), with EFI and a sleek design.

jimh--Thanks.

Heard back from the current owner and he said it has no problem getting on plane with two adults and a 100-lbs dog. He didn't specify weight of adults, nor was I going to ask. Of course, he didn't specify how long it took to get on plane, but that's reassuring enough to go check it out in person.

jimh wrote: You do not give any information about the distance of your ferry trips. The distance will affect how significant a change in boat speed might be. For example, if the distance to be traveled is only 0.5-miles, then the duration of the trip will not be much changed if the speed changes by a certain amount.

Distance from boat launch to cottage is ~1.3 miles, so yeah... top speed isn't really a concern.

When it comes to loading down with a bunch of people, I'm resigned to the fact it'll be pushing it. However owner said even with four adults he's able to get on plane it two people get cosy on the bow.

I hope a re-power isn't in my future, but just in case it is I've spent the better part of two days going through every applicable 15-foot hull re-power thread here.

Many thanks,
JB