“Always Buying, Consigning and Selling Queensland’s Best Used Boats Since 1964”

Horses for courses – issues concerning overpowering or under powering your boat. We discuss the repercussions of both and how to determine if the boat suffers from either problem.

One of the many important checks that the team at John Crawford Marine do before buying or consigning a rig is check whether the boat is either over powered or underpowered. Basically a hull is manufactured after a designer has carefully considered the many variables that have gone into the design of the hull. For example if the hull is to be used as a ski boat, the boat will have a very strong transom, sturdy internal design, all to handle high speed and a heavy, high performance engine. If the rig is a lightweight aluminium 12-foot dinghy, then the designer will also recommend a certain weight and horsepower rating relevant to its intended use.

The way to check if the hull is overpowered is by locating the manufactures plate. This plate is usually found near the transom, either high in the engine well or inside the cockpit high on the gunwale near the steering position. The plate is commonly made of an aluminium material, silver in colour and is approximately 10cm long by 6cm high. The manufacturer’s plate also has the Hull Identification Number (HIN) recorded, date of manufacture, number of persons the hull is recommended to handle and also the recommended horsepower and weight of the engine that the boat can safely handle within normal recreational boating limits.

An example of the information provided by the Hull manufacturer



An example of the information provided by the Hull manufacturer


For example if the boat is a 12 foot dinghy, the plate may read Horsepower (Hp) max rating is 25Hp or 18.6 Kilowatts (Kw) and max weight of motor not to exceed 45Kg. Digressing for a moment, the kilowatts rating term is becoming more and more popular when talking about power for a hull. I now understand what the generation before me had to endure when learning to go from imperial measurements to metric! To convert Horsepower to kilowatts simply divide horsepower by the factor 1.341

If the owner has fitted a motor which exceeds the Hp / Kw or weight rating for that hull then there are a number of behavioural characteristics that the boat may do whilst on the water. One of the first you may encounter is when launching the boat at the boat ramp water may come over the transom when sliding the boat into the water. This will happen if there is too much weight at the stern of the rig (including eskies, fuel tanks and batteries) and it is not the best way to start your day on the water! A distinctive characteristic if the boat is overpowered is the skipper will have difficulty in trimming the boat onto the plane and preventing the boat from excessive purposing. This name was given to the boat as it tends to behave like a porpoise as the bow wont stay level in the water – it continually bobs up and down whilst travelling through the water (the waves have nothing to do with this action, it will happen even in the calmest of waters).

Another trait of a hull that is over powered or as result of too much weight at the stern is when the vessel powers down off the plane to an idle speed the following stern wave will catch up to the transom of the rig and will quite literally wash in. The owner may also experience when in offshore conditions that when drifting the waves may wash into the outboard well. This also is also precarious situation to find one self in!

Apart from endangering yourself and companions on board (excessive speed that the hull can safely handle and the points already discussed), an overpowered boat is not insurable. I have often found that people have been able to insure their boat as the insurance companies (as they write in their fine print, put the onus on you as the provider of the information) do not seem to bother checking the horsepower versus the hull type. They will be happy to take your annual premium but if the time ever comes to make a claim and the assessor notes that the hull is not rated to the horsepower bolted on the transom – they will immediately make your policy null and void.

John Crawford Marine is founding and proud member of Marine Queensland

The real issue here is not so much a small amount of money to fix a broken propeller or blown up engine, but more to the point what if the overpowered rig runs into another boat whilst out of control and hurts or kills someone? Then the owner of the boat has no insurance and will certainly loose everything they own….. Again, this is why I insist that boat buyers purchase their rig through a well-respected boat yard that is a Marine Queensland member or a member Boating Industry of Australia (BIA).

But not only can hulls be overpowered but also sometimes they can be underpowered! Underpowered boats can be harder to pick because it can sometimes come back to the purchasers intended use. For example if a purchaser wants to do some occasional water skiing on a single ski, then the power to weight ratio of a 60Hp on a 5.0m runabout would not be adequate. Although the manufacturer / dealer has powered the rig adequately. An analogy would be if you wanted to go up the beach occasionally any 4WD would do but if you wanted to tow a 6.0m boat you would need a 4WD that could do the job.

An underpowered boat once on the water with a normal load takes invariably a rather long time to get on the plane. The skipper generally has to give the engine full power and maintain high revs to maintain the speed. This means the engine is working outside its fuel economical range and as a result, fuel consumption will go through the roof, not mention excessive engine wear and noise.

There are many differing designs of engine foils but they have the same principle

A means of trying to get the boat on the plane quicker is the fitting of an engine foil. You will see in the marketing of such products that they will save x number of percent in fuel, make the hull ride better, stop engine cavitation (and they may even throw in some steak knives – had to add that!) However I have found that these foils are generally added to try and hide a flaw with the boat i.e. underpowered, engine has been fitted up incorrectly or the hull is simply not up to par! I must make note that some foils do work on some hulls but its not a given that they work on all boats!

A further issue that also need considering is the resale of the boat. JCM often finds that if a boat is deemed to be underpowered by the market, it is often very hard to achieve a sale. For example in the early to mid 90’s Cruise Craft manufactured a small 15 foot fibreglass runabout called a Spirit 470SR. This boat sold in large numbers and still remains very popular on today’s second hand market. The boat manufactures plate indicated minimum Hp of 50 and a maximum of 90. The boat is set up with a forward steering configuration, drivers and passenger seat and two rear stern quarter seats. I have seen these boats with 4 adults on board and when powered with a 50Hp they can barley achieve planing speed. Yet with a 90Hp on board they are bordering on being to fast in inexperienced hands! Ideal power is a 70Hp or a 75Hp motor.


A Cruise Craft Spirit hull was rated for motors between 50 Hp and 90 Hp

If you are looking at buying a new boat either at a boat show or from a dealership bear in mind that a means of attracting a prospective customer is price. It is often the case that within the larger cities there may more than one dealer selling the same brand hull. In order to make their rig look cheaper they may sacrifice a more suitable sized engine for a slightly lower rated horsepower or model, therefore lowering the retail price and undercutting the competitor. This only leads to one person getting the raw end of the deal – the purchaser.


Buyer beware at Boat Shows, ensure your purchase is subject to a satisfactory on water demonstration

In order to prevent you, the purchaser from making a misinformed or misdirected purchase. I encourage you to make your purchase subject to a “personally satisfactory on water demonstration”. JCM always makes a water test available to our clients not only to demonstrate launch and retrieval techniques at the ramp, but also how to plane the hull with trim and tilt and basic boat handling skills. The on water demonstration will allow you the ability to feel if the boat is suited to your intended use. Then you can make your own decision. I see and hear far too often that a customer spent a lot of their hard earned money trusting a sales person who talks up their experience, only to have the customer find after a few outings that the rig isn’t quite what they wanted.

There are many boats still out their that were either made before manufacturers put the recommended information on the hulls and also some boats may have been refitted and the original plate may be missing. If this is the case and you want to know if the rig you are looking at buying or the rig you already own is overpowered, I suggest you do one off the following. Get in contact with the manufacturer if they still exist and ask them the question. Contact the dealership that sells or sold that brand – again they may have the information on hand. Or finally do some research on the Internet, several sites including www.redbook.com.au (marine section) identifies numerous boats not only their power but also dimensions, weights, and if of interest their price!

I hope some of the points raised here within help you in making and informed decision.

Safe and enjoyable boating! It’s a great pastime!

Matthew Hodson
Managing Director