One opinion is that the "longboats" of the Gaspee attackers were larger than commonly used in re-creations today of the Gaspee attack.

Illustrations of Boats Probably Used.

The longboats probably were like the ones shown in the famous painting by Charles DeWolf Brownell (Rhode Island Historical Society).  Note the size of the boats easily accommodates more than 15 persons in each boat. 

Several lines of reasoning lead to an estimate that the Gaspee raiders used long-boats well over 20 feet long --- more likely about 30 feet long --- and easily capable of holding at least 11 to 15 men.  An example of the type of long-boat probably used is the reconstructed long boat shown in the photo on the right, which boat is well over 20 feet long and is shown with 11 persons in it, and obviously not full.

Bowen's Description of the Longboats. 
We all start with the description given by Bowen: 

"Mr. Brown immediately resolved on her destruction, and he forthwith directed one of his trusty shipmasters to collect eight of the largest long-boats in the harbor, with five oars to each..."

"Five oars" probably means 5 pairs of oars
Bowen's 18th century description is a strange description to modern city dwellers.  "Five oars" is an odd number, not an even number of oars, and we assume that rowing takes an even number of oars.  At first we think that Bowen wants to tell us that the fifth oar was used for steering  (which does not make much sense because long-boats would commonly be steered by a tiller) Then we think that Bowen wants to tell us the fifth oar was to be a spare. (That also does not make much sense, because the persons whom he was addressing really would not care whether a spare oar was carried). And since we do not know by experience what an ordinary long boat of the day was, and cannot picture in our mind the size involved --- "largest" doesn't seem to help us much without some further digging. Most of us today stop after reading Bowen's description, and we do not spend time thinking about what it means.

If the audience is composed of 18th century persons familiar with ship's boats use --- "five oars" does say something about the size of the boats.  And, on close examination, Bowen's "largest" description does tell us something  about the size of the boats.

In the 18th Century,  the nautical term of "oars" when describing a boat,  could mean either "single oars" or "pairs of oars".  For an example of the use "oars" to mean "pair of oars" , see the Table of Boats carried by British vessels of war by William Mountaine, The Seaman's Vade-Mecum (London, 1757).  British warships carried a variety of small boats, for different purposes.  Mountaine describes the oars of the various "Long Boats" as e.g., "7 oars" or 8 oars"  Yet, if you look at the same type of long-boats in the British Navy drawings,  you will see the oars set out as "7 Pair" or  "8 Pair".  E.g., "18th Century Longboats After Chapman", at Notes on 18th Century Ship's Boats, Vol.  26, Nautical Research Journal, p 209 et seq. (Nautical Research Guild, Washington, D.C., Dec., 1980). So if you were in the merchant service, or in the navy,  "a  longboat with five oars,  meant the largest boat a ship could carry (see continuation page, below) with five sets of oars.

So, to sum up, if you were in the navy or on most merchant ships, you used a "longboat" which if it was said to have "five oars", it meant five sets of oars. The size of the longboat varied according to the size of the ship it was on; the larger the ship, the larger the ship's longboat. 

The only sort of boat that might be  described as having the odd number of five oars was a "whaleboat" which had six oars but only five of which were used primarily for rowing. The whaling industry in 1772 was sufficiently developed so that whaleboats had a common size and design: 28 feet long and six feet wide  The bigger the whaling ship, the more whaleboats, not larger boats.  Because the more whale boats that could be launched the more whales could be caught, and the whaling ships of the need had limited space to carry crew and house and feed the crew, the whaleboat had to be rowed by a limited number of men, and the boats had to be limited in size.  See, Willits D. Ansel, The Whaleboat (Mystic Seaport Museum, 1978). Hence, a whaleboat would almost always be 28 feet long and six feet wide, no matter how big the ship used for the whaling enterprise.  Because of the configuration of the unusual oars and oarlocks used in a whaleboat, there was room for only six or seven men..

In contrast, the longboats of merchant ships varied in size according to the size of the ship, and were designed to carry many men or substantial cargo. Merchant ships of the day usually carried one long boat (a "longboat"  designed to carry merchandise to and from the ship, and to be a substantial lifeboat if the ship sank.  The standard formula for the "longboat" (largest boat carried on the ship) was figured by taking the square root of the length of the ship and multiplying that square root number by 2.6.  A sloop's long boat would not be unusual at 28 or so feet in length. 

Bowen referred to all the boats as "longboats" and not as "whaleboats" which strongly suggest that he meant boats with five sets of oars, not whaleboats with their unusual configuration and limited space for men to be transported.  Mawney referred to his boat as a "barge". A barge was included in the general term of "longboat" as meaning the largest boat carried by the merchant ships and navy ships. A ship's barge was the type of big boat found only on ships of the line and other large navy ships.  It not only was used for carrying the ranking officer, but also for the landing of marines and troops. A barge would ordinarily note be handled by five single oars, but rather by five sets of oars. The "barge" term tends to lead to the conclusion that the boats probably had five "sets" of oars.

We can deduce a minimum size for the longboats from the necessary size of a rowed boat with 5 pairs of oars, which need at least 3 feet of length for each set of oars and pair of rowers.  Therefore, considering some room at the bow and some at the stern, in addition to the space for 5 seats for rowing, the longboats must have been in excess of 18 feet long.

But there is more.  By 1750 shipbuilding in Rhode Island had reached the levels of sophistication of England.  There were common formulas used to determine various aspects of the ship once the length of the ship was agreed upon between purchaser and builder. This included the length and size of the ship's longboat. 

Read on to find that Bowen's description of "largest long-boats" would have meant, to the persons he was addressing, boats of  more than 25 feet long.  Discussion continued about the size of the longboats in the Gaspee attack.